Supplemental Prospectus (Form 424B5)

Filed Pursuant to Rule 424(b)(5)
File No. 333-291167

TABLE OF CONTENTS

Prospectus Supplement

Prospectus

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part, this prospectus supplement, describes the specific terms of this offering of the Notes and certain other matters relating to NiSource Inc. The second part, the accompanying prospectus, gives more general information, some of which does not apply to this offering. To the extent there is a conflict or inconsistency between the information contained or incorporated by reference in this prospectus supplement (or any related free writing prospectus issued by us), on the one hand, and the information contained or incorporated by reference in the accompanying prospectus, the information contained or incorporated by reference in this prospectus supplement (or any related free writing prospectus issued by us) shall control.

The registration statement of which this prospectus supplement and the accompanying prospectus form a part, including certain exhibits to the registration statement, provides additional information about us and the Notes offered under this prospectus supplement and the accompanying prospectus. Specifically, we have filed with the Securities and Exchange Commission ("SEC") and incorporated by reference, and may in the future file and incorporate by reference, certain legal documents that control the terms of the Notes offered by this prospectus supplement and the accompanying prospectus as exhibits to the registration statement, as described under "Description of the Debt Securities" in the accompanying prospectus.

This prospectus supplement, the accompanying prospectus and certain of the documents incorporated by reference herein and therein contain, and any related free writing prospectus issued by us may contain, summaries of information contained in documents that we have filed or will file as exhibits to our SEC filings. Such summaries do not purport to be complete and are subject to, and qualified in their entirety by reference to, the actual documents filed with the SEC.

You should read this entire prospectus supplement and the accompanying prospectus, including the documents incorporated by reference in this prospectus supplement under "Incorporation By Reference," and any related free writing prospectus provided in connection with this offering before deciding whether to invest in any Notes offered by this prospectus supplement. We have not, and the underwriters have not, authorized anyone to provide you with different or additional information, other than as described in the preceding sentence. We and the underwriters take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give to you. This prospectus supplement and the accompanying prospectus may only be used where it is legal to sell the Notes offered hereby. The information in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein is accurate only as of the date of the respective documents in which the information appears. Our business, financial condition, results of operations and prospects may have changed since those dates, and neither the delivery of this prospectus supplement and the accompanying prospectus nor any sale hereunder shall, under any circumstances, create any implication to the contrary.

When we refer to "NiSource," "we," "our," "ours" and "us" in this prospectus supplement under the heading "Forward-Looking Statements" we mean NiSource Inc. and its subsidiaries, through which substantially all of NiSource Inc.'s operations are conducted. When such terms are used elsewhere in this prospectus supplement, we refer only to NiSource Inc., as the issuer of securities in this offering, and not to any of its direct or indirect subsidiaries or affiliates except as expressly provided or the context otherwise requires.

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FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus, and the information incorporated by reference herein and therein, include forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended (the "Securities Act"), and Section 21E of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). Forward-looking statements are based on management's beliefs and assumptions and can often be identified by terms and phrases that include, "may," "will," "should," "could," "would," "aims," "seeks," "expects," "plans," "projects," "anticipates," "intends," "believes," "estimates," "predicts," "potential," "targets," "forecast," "guidance," "outlook," and "continue," or other similar terminology. Various factors may cause actual results to be materially different than the suggested outcomes within forward-looking statements; accordingly, there is no assurance that such results will be realized. These factors include, but are not limited to:

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For more information about the significant risks that could affect the outcome of these forward-looking statements and our future financial condition, results of operations, liquidity and cash flows, you should read the sections titled "Risk Factors" in the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, together with "Risk Factors" in this prospectus supplement. In light of these risks, uncertainties and assumptions, the events described in the forward-looking statements included or incorporated by reference in this prospectus supplement and the accompanying prospectus might not occur or might occur to a different extent or at a different time than described. We qualify all of our forward-looking statements by these cautionary statements. Forward-looking statements speak only as of the date they are made, and we expressly disclaim any obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, except as may be required by law.

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WHERE YOU CAN FIND MORE INFORMATION

We are subject to the informational requirements of the Exchange Act, and, in accordance therewith, file annual, quarterly and current reports, proxy statements and other information with the SEC. Our filings with the SEC, as well as additional information about us, are available to the public through our website at http://www.nisource.com and are made available as soon as reasonably practicable after such material is filed with or furnished to the SEC. The information on our website is not a part of this prospectus supplement or the accompanying prospectus. Our filings are also available to the public through the SEC's website at http://www.sec.gov.

INCORPORATION BY REFERENCE

The SEC allows us to "incorporate by reference" information into this prospectus supplement and the accompanying prospectus. This means that we can disclose important business, financial and other information to you by referring you to another document that NiSource has filed separately with the SEC. The information incorporated by reference is considered to be part of this prospectus supplement and the accompanying prospectus. Information that NiSource files with the SEC after the date of this prospectus supplement will automatically modify and supersede the information included or incorporated by reference in this prospectus supplement and the accompanying prospectus to the extent that the subsequently filed information modifies or supersedes the existing information. We incorporate by reference:

You may request a copy of any of these filings at no cost by writing to or calling us at the following address and telephone number: Corporate Secretary, NiSource Inc., 801 East 86th Avenue, Merrillville, Indiana 46410, telephone: (614) 460-6000.

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RISK FACTORS

Investing in the Notes involves risks. You should read carefully the information contained elsewhere or incorporated by reference in this prospectus supplement and the accompanying prospectus and should carefully consider the following risk factors, as well as the "Risk Factors" and "Forward-Looking Statements" sections in the accompanying prospectus and the "Risk Factors" and "Note regarding forward-looking statements" sections in our most recent Annual Report on Form 10-K, which is incorporated by reference herein, and in any subsequent periodic reports that are incorporated by reference into this prospectus supplement and the accompanying prospectus. Each of the risks described could materially adversely affect our operations and financial results and the value of the Notes and your investment therein.

Risks Relating to the Notes and this Offering

We may redeem each series of the Notes if a Tax Credit Event occurs.

We may redeem each series of the Notes, at our option, in whole but not in part, if a Tax Credit Event occurs. A "Tax Credit Event" occurs with respect to a series of the Notes if, in our reasonable determination, there exists a material risk, due to the Notes (considered together with other debt) having been issued, as part of an original issuance, to one or more "specified foreign entities," as defined in Section 7701(a)(51)(B) of the Internal Revenue Code of 1986, as amended (the "Code"), that we or any of our affiliates would be unable to utilize or otherwise ineligible to claim any tax credits otherwise allowed under Section 38 of the Code. "Specified foreign entities," as further defined in Section 7701(a)(51)(B) of the Code, generally include, among other entities: (i) the governments of China, Iran, North Korea or Russia or their agencies or instrumentalities, (ii) certain citizens or nationals of such countries, (iii) entities organized under the laws of, or having their principal place of business in, such countries, (iv) entities controlled by any of the above, including subsidiaries, measured by more than 50% ownership of stock (by vote or value) in a corporation, profits interests or capital interests in a partnership, or beneficial interest in another entity, (v) certain Chinese military companies described under Section 1260H of the William M. (Mac) Thornberry National Defense Authorization Act for Fiscal Year 2021, (vi) Contemporary Amperex Technology Company (CATL), BYD Company, Envision Energy, EVE Energy Company, Gotion High-tech Company, Hithium Energy Storage Technology Company, or any successor company to the foregoing, (vii) certain companies that violate the Uyghur Forced Labor Prevention Act of 2021, and (viii) entities that the Office of Foreign Assets Control of the Department of the Treasury ("OFAC") have included on the list of specially designated nationals and blocked persons maintained by OFAC. A redemption of a series of the Notes for this reason would be at a redemption price equal to 101% of the principal amount of the Notes being redeemed, in each case plus accrued and unpaid interest to but excluding the redemption date.

If a Tax Credit Event occurs, and we redeem the Notes of a particular series, such redemption may adversely affect your anticipated return. We may exercise such redemption rights when prevailing interest rates are relatively low. If prevailing interest rates are lower at the time of redemption, you may not be able to reinvest the redemption proceeds in a comparable security at an interest rate as high as the effective interest rate of the Notes being redeemed. See the "Supplemental Description of the Notes-Tax Credit Event Redemption" section of this prospectus supplement for more information.

The Notes are obligations of NiSource and not of NiSource's subsidiaries and will be structurally subordinated to the claims of such subsidiaries' creditors.

The Notes are obligations of NiSource Inc., and not of any of NiSource's subsidiaries. NiSource is a holding company and, accordingly, we conduct substantially all of our operations through our operating subsidiaries. As a result, our cash flow and our ability to service our debt, including the Notes, depend upon the earnings of our operating subsidiaries and on the distribution of earnings, loans or other payments by such subsidiaries to NiSource.

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Our operating subsidiaries are separate and distinct legal entities and have no obligation to pay any amounts due on the Notes or to provide us with funds for our payment obligations, whether by dividends, distributions, loans or other payments. In addition, any payment of dividends, distributions, loans or advances by our subsidiaries to us could be subject to statutory or contractual restrictions. Payments to us by our operating subsidiaries will also be contingent upon such subsidiaries' earnings and business considerations. As of March 31, 2026, our operating subsidiaries had, in addition to other liabilities, approximately $0.3 billion aggregate amount of indebtedness.

Our right to receive any assets of any of our subsidiaries upon their liquidation or reorganization, and therefore the rights of the holders of the Notes to participate in those assets, will be structurally subordinated to the claims of that subsidiary's creditors. In addition, even if we were a creditor of any of our subsidiaries, our rights as a creditor would be subordinate to any security interest in the assets of our subsidiaries and any indebtedness of our subsidiaries senior to that held by us. There is no limitation on the ability of our subsidiaries to incur additional indebtedness to which the Notes will be structurally subordinated. Additionally, if any of our subsidiaries were to issue preferred stock in the future, the Notes would similarly be structurally subordinated to the rights of the preferred stockholders.

See "Risk Factors-Financial, Economic and Market Risks-We are a holding company and are dependent on cash generated by our subsidiaries to meet our debt obligations and pay dividends on our stock" in our Annual Report on Form 10-K for the fiscal year ended December 31, 2025 incorporated by reference in this prospectus supplement and the accompanying prospectus.

The terms of the Indenture and each series of the Notes do not provide protection against certain significant events that could adversely impact a holder's investment in the Notes.

The terms of the Indenture and each series of the Notes do not restrict our ability to engage in, or to otherwise be a party to, a variety of corporate transactions, circumstances and events that could have an adverse impact on an investment in the Notes. In particular, the Indenture does not:

There is no guarantee that an active trading market for the Notes will exist or that you will be able to sell your Notes.

Each series of Notes will constitute a new issuance of securities without an established trading market and a market may not develop for the Notes of such series. As a result, you may not be able to sell your Notes. Accordingly, you may be required to bear the financial risk of an investment in the Notes for an indefinite period of time. In addition, the condition of the financial markets and prevailing interest rates have fluctuated in the past and are likely to fluctuate in the future, which could have an adverse effect on the market prices of the Notes. We have been advised by the underwriters that they may make a market in the Notes, but they have no obligation to do so and may discontinue market making at any time without providing notice. There can be no assurance that a

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market for the Notes of either series will develop or, if it does develop, that it will continue. If an active public market does not develop or does not continue, the market prices and liquidity of such Notes may be adversely affected.

Furthermore, we do not intend to apply for listing of the Notes on any securities exchange or seek their quotation on any automated dealer quotation system.

If trading markets do develop, changes in our ratings or the financial markets could adversely affect the market prices of the Notes.

The market prices of the Notes will depend on many factors, including, among others, the following:

The condition of the financial markets and prevailing interest rates have fluctuated in the past and are likely to fluctuate in the future, which could have an adverse effect on the market prices of the Notes. In general, as market interest rates rise, debt securities bearing interest at fixed rates decline in value. Consequently, if market interest rates increase, the market value of your Notes may decline. We cannot predict the future level of market interest rates.

Rating agencies continually review the ratings they have assigned to companies and debt securities. Negative changes in the ratings assigned to us or our debt securities could have an adverse effect on the market prices of the Notes.

Our credit ratings may not reflect all risks of your investments in the Notes.

Our credit ratings are an assessment by rating agencies of our ability to pay our debts when due. Consequently, real or anticipated changes in our credit ratings will generally affect the market value of the Notes. These credit ratings may not reflect the potential impact of risks relating to the structure or marketing of the Notes. Agency ratings are not a recommendation to buy, sell or hold any security, and may be revised or withdrawn at any time by the issuing organization. Each agency's rating should be evaluated independently of any other agency's rating.

We may choose to redeem either series of the Notes prior to maturity.

In addition to redemption upon a Tax Credit Event, we may redeem all or a portion of either series of the Notes at our option at any time at the applicable redemption price described in this prospectus supplement. See "Supplemental Description of the Notes-Optional Redemption". We may choose to redeem your Notes at a time when prevailing interest rates are lower than the interest rate paid on your Notes. If prevailing interest rates are lower at the time of redemption, you may not be able to reinvest the redemption proceeds in a comparable security at an interest rate as high as the effective interest rate of the Notes being redeemed.

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USE OF PROCEEDS

The aggregate net proceeds to us from the sale of the Notes, after deducting the underwriting discounts but before deducting our other estimated fees and expenses related to this offering, will be approximately $    .

We intend to use the aggregate net proceeds from the sale of the Notes for general corporate purposes, including to finance capital expenditures, for working capital and to repay existing indebtedness.

Affiliates of certain of the underwriters are lenders under our revolving credit facility. To the extent that we use the aggregate net proceeds from this offering to repay amounts we have borrowed or may borrow or re-borrow in the future under our revolving credit facility, these lenders will receive their pro rata portions of such proceeds. The term of our revolving credit facility expires on December 11, 2030. In addition, certain of the underwriters are dealers under our commercial paper program. To the extent we use the aggregate net proceeds from this offering to repay notes issued under our commercial paper program and such underwriters hold such notes, such underwriters will receive proceeds from this offering. See the "Underwriting (Conflicts of Interest)-Conflicts of Interest" section of this prospectus supplement for more information.

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SUPPLEMENTAL DESCRIPTION OF THE NOTES

The following description summarizes certain terms applicable to the Notes and is not intended to be a complete recitation of all terms applicable to the Notes. Please read the following information concerning the Notes in conjunction with the statements under "Description of the Debt Securities" in the accompanying prospectus, which the following information supplements and, if there are any inconsistencies, supersedes. The Notes will be issued under the Indenture, dated as of November 14, 2000, as amended and supplemented by the Second Supplemental Indenture, dated as of November 30, 2017 (collectively, the "Indenture"), each between NiSource Inc. and The Bank of New York Mellon, as successor trustee. The Indenture is described in the accompanying prospectus and is filed as an exhibit to the registration statement under which the Notes are being offered and sold.

Maturity, Interest and Payment

2031 Notes

The 2031 Notes will mature on     , 2031, subject to earlier redemption at our option as described under "-Optional Redemption-2031 Notes" and "-Tax Credit Event Redemption."

The 2031 Notes will bear interest at a rate of    % per annum from and including    , 2026, payable semi-annually in arrears on      and      of each year, beginning     , 2026. Interest payable on each interest payment date for the 2031 Notes will be paid to the persons in whose name the 2031 Notes are registered at the close of business on the record date for the applicable interest payment date, which will be (i) the business day immediately preceding such interest payment date so long as all of the 2031 Notes remain in book-entry only form or (ii) each      and      (whether or not a business day) if any of the 2031 Notes do not remain in book-entry only form.

If an interest payment date falls on a day that is not a business day, interest will be payable on the next succeeding business day with the same force and effect as if made on such interest payment date, with no additional interest accruing as a result of such postponed payment. Interest on the 2031 Notes will be calculated on the basis of a 360-day year, consisting of twelve 30-day months.

2036 Notes

The 2036 Notes will mature on     , 2036, subject to earlier redemption at our option as described under "-Optional Redemption-2036 Notes" and "-Tax Credit Event Redemption."

The 2036 Notes will bear interest at a rate of    % per annum from and including    , 2026, payable semi-annually in arrears on      and      of each year, beginning     , 2026. Interest payable on each interest payment date for the 2036 Notes will be paid to the persons in whose name the 2036 Notes are registered at the close of business on the record date for the applicable interest payment date, which will be (i) the business day immediately preceding such interest payment date so long as all of the 2036 Notes remain in book-entry only form or (ii) each      and      (whether or not a business day) if any of the 2036 Notes do not remain in book-entry only form.

If an interest payment date falls on a day that is not a business day, interest will be payable on the next succeeding business day with the same force and effect as if made on such interest payment date, with no additional interest accruing as a result of such postponed payment. Interest on the 2036 Notes will be calculated on the basis of a 360-day year, consisting of twelve 30-day months.

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Optional Redemption

2031 Notes

Prior to     , 2031 (    month prior to the maturity date of the 2031 Notes) (the "2031 Notes Par Call Date"), we may redeem the 2031 Notes, at our option, in whole or in part, at any time and from time to time, at a redemption price (expressed as a percentage of principal amount and rounded to three decimal places) equal to the greater of:

(1)(a) the sum of the present values of the remaining scheduled payments of principal and interest thereon discounted to the redemption date (assuming the 2031 Notes matured on the 2031 Notes Par Call Date) on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate (as defined below) applicable to the 2031 Notes plus  basis points, less (b) interest accrued to the date of redemption, and

(2) 100% of the principal amount of the 2031 Notes to be redeemed,

plus, in either case, accrued and unpaid interest thereon to, but excluding, the redemption date.

On or after the 2031 Notes Par Call Date, we may redeem the 2031 Notes at our option, in whole or in part, at any time and from time to time, at a redemption price equal to 100% of the principal amount of the 2031 Notes being redeemed plus accrued and unpaid interest thereon to, but excluding, the redemption date.

2036 Notes

Prior to     , 2036 (     months prior to the maturity date of the 2036 Notes) (the "2036 Notes Par Call Date"), we may redeem the 2036 Notes, at our option, in whole or in part, at any time and from time to time, at a redemption price (expressed as a percentage of principal amount and rounded to three decimal places) equal to the greater of:

(1)(a) the sum of the present values of the remaining scheduled payments of principal and interest thereon discounted to the redemption date (assuming the 2036 Notes matured on the 2036 Notes Par Call Date) on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Rate applicable to the 2036 Notes plus  basis points, less (b) interest accrued to the date of redemption, and

(2) 100% of the principal amount of the 2036 Notes to be redeemed,

plus, in either case, accrued and unpaid interest thereon to, but excluding, the redemption date.

On or after the 2036 Notes Par Call Date, we may redeem the 2036 Notes at our option, in whole or in part, at any time and from time to time, at a redemption price equal to 100% of the principal amount of the 2036 Notes being redeemed plus accrued and unpaid interest thereon to, but excluding, the redemption date.

For purposes of these redemption provisions relating to either series of the Notes, the following terms have the following meanings:

"Treasury Rate" means, with respect to any redemption date for any of the 2031 Notes or the 2036 Notes, as applicable, the yield determined by us in accordance with the following two paragraphs. The Treasury Rate shall be determined by us after 4:15 p.m., New York City time (or after such time as yields on U.S. government securities are posted daily by the Board of Governors of the Federal Reserve System), on the third business day preceding the redemption date based upon the yield or yields for the most recent day that appear after such time on such day in the most recent statistical release published by the Board of Governors of the Federal Reserve System designated as "Selected Interest Rates (Daily)-H.15" (or any successor designation or publication) ("H.15") under the caption "U.S. government securities-Treasury constant maturities-Nominal" (or any successor caption or heading) ("H.15 TCM"). In determining the Treasury Rate, we shall select, as applicable:

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(1) the yield for the Treasury constant maturity on H.15 exactly equal to the period from the redemption date to the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable (the "Remaining Life"); or (2) if there is no such Treasury constant maturity on H.15 exactly equal to the Remaining Life, the two yields-one yield corresponding to the Treasury constant maturity on H.15 immediately shorter than and one yield corresponding to the Treasury constant maturity on H.15 immediately longer than the Remaining Life-and shall interpolate to the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, on a straight-line basis (using the actual number of days) using such yields and rounding the result to three decimal places; or (3) if there is no such Treasury constant maturity on H.15 shorter than or longer than the Remaining Life, the yield for the single Treasury constant maturity on H.15 closest to the Remaining Life. For purposes of this paragraph, the applicable Treasury constant maturity or maturities on H.15 shall be deemed to have a maturity date equal to the relevant number of months or years, as applicable, of such Treasury constant maturity from the redemption date.

If on the third business day preceding the redemption date H.15 TCM is no longer published, we shall calculate the Treasury Rate based on the rate per annum equal to the semi-annual equivalent yield to maturity at 11:00 a.m., New York City time, on the second business day preceding such redemption date of the United States Treasury security maturing on, or with a maturity that is closest to, the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable. If there is no United States Treasury security maturing on the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, but there are two or more United States Treasury securities with a maturity date equally distant from the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, one with a maturity date preceding the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, and one with a maturity date following the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, we shall select the United States Treasury security with a maturity date preceding the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable. If there are two or more United States Treasury securities maturing on the 2031 Notes Par Call Date or the 2036 Notes Par Call Date, as applicable, or two or more United States Treasury securities meeting the criteria of the preceding sentence, we shall select from among these two or more United States Treasury securities the United States Treasury security that is trading closest to par based upon the average of the bid and asked prices for such United States Treasury securities at 11:00 a.m., New York City time. In determining the Treasury Rate in accordance with the terms of this paragraph, the semi-annual yield to maturity of the applicable United States Treasury security shall be based upon the average of the bid and asked prices (expressed as a percentage of principal amount) at 11:00 a.m., New York City time, of such United States Treasury security, and rounded to three decimal places.

Our actions and determinations in determining the redemption price shall be conclusive and binding for all purposes, absent manifest error.

The trustee shall have no duty to determine, or verify the calculation of, the redemption price.

Tax Credit Event Redemption

If a Tax Credit Event (as defined below) occurs, we may redeem, upon a notice of redemption, the Notes of either series, as applicable, in whole but not in part at a redemption price equal to 101% of the principal amount of the Notes being redeemed, plus accrued and unpaid interest to but excluding the redemption date. A notice of redemption of the Notes of either series upon the occurrence of a Tax Credit Event (i) may only be sent by the later of (a) the end of the calendar year in which the Notes of such series were issued and (b) six months from the date of issuance of the Notes of such series and (ii) shall be accompanied by a certificate from an officer of NiSource stating that a Tax Credit Event has occurred.

The consummation of a redemption upon a Tax Credit Event may be subject to the paying agent's receipt of the required redemption moneys on or before the redemption date (and in such case no such redemption shall occur unless such moneys have been received by the paying agent on or before such date).

A "Tax Credit Event" occurs with respect to a series of the Notes if, in our reasonable determination, there exists a material risk, due to Notes (considered together with other debt) having been issued, as part of an

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original issuance, to one or more "specified foreign entities," as defined in Section 7701(a)(51)(B) of the Code, that we or any of our affiliates would be unable to utilize or otherwise ineligible to claim any tax credits otherwise allowed under Section 38 of the Code.

Selection and Notice of Redemption

Notice of any redemption will be mailed or electronically delivered (or otherwise transmitted in accordance with the depositary's procedures) at least 10 days but not more than 60 days before the redemption date to each holder of the applicable Notes to be redeemed.

In the case of a partial redemption, if the Notes to be redeemed are held in certificated form, selection of the Notes for redemption will be made by the trustee, by lot. No Notes of a principal amount of $2,000 or less will be redeemed in part. If either series of the Notes is to be redeemed in part only, the notice of redemption that relates to such series of the Notes will state the portion of the principal amount of such series of the Notes to be redeemed. A new Note in a principal amount equal to the unredeemed portion of the original Note will be issued in the name of the holder of the original Note upon surrender for cancellation of the original Note. For so long as the Notes are held by DTC (or another depositary), the redemption of the Notes, including selection of the Notes for redemption, shall be done in accordance with the policies and procedures of the depositary and may be made on a pro rata pass-through distribution of principal basis.

Unless we default in payment of the redemption price, on and after the redemption date interest will cease to accrue on the Notes or portions thereof called for redemption.

Additional Notes

We may, without the consent of the holders of either series of the Notes, create and issue additional 2031 Notes or 2036 Notes ranking equally with such series of the Notes in all respects, including having the same terms (except for the price to public, the issue date, the initial interest accrual date and the first interest payment date, as applicable) as the 2031 Notes or the 2036 Notes, as applicable, so that such additional Notes would be consolidated and form a single series with the 2031 Notes or the 2036 Notes, as applicable, and would have the same terms as to status, redemption or otherwise as such series of the Notes. Such additional Notes will have the same CUSIP number as the 2031 Notes or the 2036 Notes, as applicable, offered hereby, provided that such additional Notes must be part of the same issue as such series of the Notes offered hereby for U.S. federal income tax purposes or, if they are not part of the same issue for such purposes, such additional Notes must be issued with a separate CUSIP number. No additional Notes may be issued if an Event of Default under the Indenture has occurred and is continuing with respect to the 2031 Notes or the 2036 Notes, as applicable.

Forms and Denominations

Each series of the Notes will be issued as one or more global securities in the name of a nominee of DTC and will be available only in book-entry form. See "- Book-Entry Only Issuance-The Depository Trust Company." The Notes will be issued in denominations of $2,000 and integral multiples of $1,000 in excess thereof.

Book-Entry Only Issuance-The Depository Trust Company

DTC will act as the initial securities depositary for the Notes. Each series of the Notes will be issued only as fully registered securities registered in the name of Cede & Co., DTC's partnership nominee, or such other name as may be requested by an authorized representative of DTC. Each series of the Notes initially will be represented by one or more fully registered global securities, representing in the aggregate the total principal amount of such series of the Notes, and will be deposited with the trustee on behalf of DTC. Investors may hold interests in the Notes through DTC if they are participants in DTC or indirectly through organizations that are participants in DTC, including Euroclear or Clearstream.

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DTC, the world's largest securities depository, is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code and a "clearing agency" registered under the provisions of Section 17A of the Exchange Act. DTC holds and provides asset servicing for U.S. and non-U.S. equity issues, corporate and municipal debt issues and money market instruments that DTC's participants ("Direct Participants") deposit with DTC. DTC also facilitates the post-trade settlement among Direct Participants of sales and other securities transactions in deposited securities, through electronic computerized book-entry transfers and pledges between Direct Participants' accounts. This eliminates the need for physical movement of securities certificates. Direct Participants include both U.S. and non-U.S. securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations. DTC is a wholly-owned subsidiary of The Depository Trust & Clearing Corporation ("DTCC"). DTCC is the holding company for DTC, National Securities Clearing Corporation and Fixed Income Clearing Corporation, all of which are registered clearing agencies. DTCC is owned by the users of its regulated subsidiaries. Access to the DTC system is also available to others such as U.S. and non-U.S. securities brokers and dealers, banks, trust companies and clearing corporations that clear through or maintain a custodial relationship with a Direct Participant, either directly or indirectly ("Indirect Participants"). The DTC rules applicable to its Direct Participants and Indirect Participants are on file with the SEC.

Purchases of Notes under the DTC system must be made by or through Direct Participants, which will receive a credit for the Notes on DTC's records. The ownership interest of each actual purchaser of Notes ("Beneficial Owner") is in turn to be recorded on the Direct Participants' and Indirect Participants' records. Beneficial Owners will not receive written confirmation from DTC of their purchases. Beneficial Owners, however, are expected to receive written confirmations providing details of the transactions, as well as periodic statements of their holdings, from the Direct Participants or Indirect Participants through which the Beneficial Owners purchased Notes. Transfers of ownership interests in the Notes are to be accomplished by entries made on the books of Direct Participants and Indirect Participants acting on behalf of Beneficial Owners. Beneficial Owners will not receive certificates representing their ownership interests in Notes, except in the event that use of the book-entry system for the Notes is discontinued.

To facilitate subsequent transfers, all Notes deposited by Direct Participants with DTC are registered in the name of DTC's partnership nominee, Cede & Co., or such other name as may be requested by an authorized representative of DTC. The deposit of Notes with DTC and their registration in the name of Cede & Co. or such other DTC nominee do not effect any changes in beneficial ownership. DTC has no knowledge of the actual Beneficial Owners of the Notes. DTC's records reflect only the identity of the Direct Participants to whose accounts such Notes are credited, which may or may not be the Beneficial Owners. The Direct Participants and Indirect Participants will remain responsible for keeping account of their holdings on behalf of their customers.

Conveyance of notices and other communications by DTC to Direct Participants, by Direct Participants to Indirect Participants, and by Direct Participants and Indirect Participants to Beneficial Owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

Redemption notices will be sent to DTC. If less than all of the Notes of either series are being redeemed, DTC's practice is to determine by lot the amount of interest of each Direct Participant in such Notes to be redeemed unless the governing documents clearly indicate that a partial redemption processed through DTC will be treated by DTC, in accordance with its rules and procedures, as a "pro rata pass-through distribution of principal."

Although voting with respect to the Notes is limited, in those cases where a vote is required, neither DTC nor Cede & Co. (nor any other DTC nominee) will consent or vote with respect to the Notes unless authorized by a Direct Participant in accordance with DTC's procedures. Under its usual procedures, DTC mails an omnibus proxy to NiSource as soon as possible after the record date. The omnibus proxy assigns Cede & Co.'s consenting

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or voting rights to those Direct Participants to whose accounts the Notes are credited on the record date (identified in a listing attached to the omnibus proxy).

Payments on the Notes will be made to Cede & Co., or such other nominee as may be requested by an authorized representative of DTC. DTC's practice is to credit Direct Participants' accounts upon DTC's receipt of funds and corresponding detail information on the relevant payment date in accordance with their respective holdings shown on DTC's records. Payments by Direct Participants or Indirect Participants to Beneficial Owners will be governed by standing instructions and customary practices, as is the case with securities held for the account of customers registered in "street name," and will be the responsibility of such Direct Participants or Indirect Participant and not of DTC, NiSource or the trustee, subject to any statutory or regulatory requirements. Payment to Cede & Co. (or such other nominee as may be requested by an authorized representative of DTC) is the responsibility of NiSource and the applicable paying agent, disbursement of such payments to Direct Participants is the responsibility of DTC, and disbursement of such payments to the Beneficial Owners is the responsibility of Direct Participants and Indirect Participants.

Except as provided herein, a Beneficial Owner of Notes will not be entitled to receive physical delivery of such Notes. Accordingly, each Beneficial Owner must rely on the procedures of DTC to exercise any rights under the Notes. The laws of some jurisdictions require that certain purchasers of securities take physical delivery of securities in definitive form. Such laws may impair the ability to transfer beneficial interests in the global Notes.

DTC may discontinue providing its services as securities depositary with respect to the Notes at any time by giving reasonable notice to NiSource or the trustee. Under such circumstances, in the event that a successor securities depositary is not obtained, the Notes certificates will be required to be printed and delivered to the holders of record.

NiSource may decide to discontinue use of the system of book-entry transfers through DTC (or a successor securities depositary) with respect to the Notes. NiSource understands, however, that under current industry practices, DTC would notify its Direct Participants and Indirect Participants of NiSource's decision, but will only withdraw beneficial interests from the global Notes at the request of each Direct Participant or Indirect Participant. In that event, certificates for the Notes will be printed and delivered to the applicable Direct Participant or Indirect Participant.

The information in this section concerning DTC and DTC's book-entry system has been obtained from sources that NiSource believes to be reliable, but neither NiSource nor any underwriter takes any responsibility for the accuracy thereof. Neither NiSource nor any underwriter has any responsibility for the performance by DTC or its Direct Participants or Indirect Participants of their respective obligations as described herein or under the rules and procedures governing their respective operations.

Global Clearance and Settlement Procedures

Secondary market trading between Clearstream participants and/or Euroclear system participants will occur in the ordinary way in accordance with the applicable rules and operating procedures of Clearstream and the Euroclear system, as applicable.

Cross-market transfers between persons holding directly or indirectly through DTC on the one hand, and directly or indirectly through Clearstream participants or Euroclear system participants on the other, will be effected through DTC in accordance with DTC rules on behalf of the relevant European international clearing system by its U.S. depositary; however, such cross-market transactions will require delivery of instructions to the relevant European international clearing system by the counterparty in such system in accordance with its rules and procedures and within its established deadlines (European time). The relevant European international clearing system will, if the transaction meets its settlement requirements, deliver instructions to its U.S.

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depositary to take action to effect final settlement on its behalf by delivering or receiving securities in DTC, and making or receiving payment in accordance with normal procedures for same-day funds settlement applicable to DTC. Clearstream participants and Euroclear system participants may not deliver instructions directly to their respective U.S. depositaries.

Because of time-zone differences, credits of Notes received in Clearstream or the Euroclear system as a result of a transaction with a DTC participant will be made during subsequent securities settlement processing and dated the business day following the DTC settlement date. Such credits or any transactions in such Notes settled during such processing will be reported to the relevant Clearstream participant or Euroclear system participant on such business day. Cash received in Clearstream or the Euroclear system as a result of sales of the Notes by or through a Clearstream participant or a Euroclear system participant to a DTC participant will be received with value on the DTC settlement date but will be available in the relevant Clearstream or the Euroclear system cash account only as of the business day following settlement in DTC.

Although DTC, Clearstream and Euroclear have agreed to the foregoing procedures to facilitate transfers of the Notes among participants of DTC, Clearstream and Euroclear, they are under no obligation to perform or continue to perform such procedures and such procedures may be changed or discontinued at any time.

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CERTAIN UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS

The following discussion summarizes certain U.S. federal income tax considerations relevant to the acquisition, ownership and disposition of Notes held as capital assets that are purchased in this offering in each case at their respective "issue price," which is the first price at which a substantial amount of each series of the Notes are sold to the public. This discussion is for general information only and does not address, or purport to address, all of the potential U.S. federal income tax considerations that may be relevant to a holder with respect to the acquisition, ownership or disposition of the Notes. Without limiting the generality of the foregoing, the discussion does not address the effect of any special rules applicable to certain types of holders, including, without limitation, traders or dealers in securities, commodities or currencies, insurance companies, financial institutions, thrifts, mutual funds, real estate investment trusts, regulated investment companies, tax-exempt entities, personal holding companies, "controlled foreign corporations," "passive foreign investment companies," U.S. Holders (defined below) whose functional currency is not the U.S. dollar, U.S. expatriates (or former long-term residents of the United States), persons who hold Notes as part of a straddle, hedge, conversion transaction, or other risk reduction or integrated investment transaction, investors in securities that elect to use a mark-to-market method of accounting for their securities holdings, individual retirement accounts or qualified pension plans, partnerships or other pass-through entities for U.S. federal income tax purposes (including Subchapter S corporations) that invest in the Notes, and persons subject to special tax accounting rules as a result of any item of gross income with respect to the Notes being taken into account in an applicable financial statement. This discussion does not address the Medicare contribution tax on net investment income or the effect of any U.S. state or local income or other tax laws, any U.S. federal estate, gift or alternative minimum tax laws, any foreign tax laws or any tax treaties. For purposes of this discussion, "holder" means either a U.S. Holder (as defined below) or a Non-U.S. Holder (as defined below) or both, as the context may require.

The discussion is based upon provisions of the Code, existing and proposed Treasury regulations promulgated thereunder, rulings, pronouncements, judicial decisions and administrative interpretations of the IRS, all as in effect as of the date of this prospectus supplement and all of which are subject to change, possibly on a retroactive basis, at any time.

NiSource has not and will not seek any rulings or opinions from the IRS with respect to the matters discussed below. There can be no assurance that the IRS will not take a different position concerning the tax consequences of the acquisition, ownership and disposition of the Notes or that any such position would not be sustained.

For purposes of this summary, a "U.S. Holder" means a beneficial owner of the Notes that, for U.S. federal income tax purposes, is:

A "Non-U.S. Holder" means a beneficial owner of the Notes (other than a partnership for U.S. federal income tax purposes) that is not a U.S. Holder.

If a partnership, or other entity or arrangement treated as a partnership for U.S. federal income tax purposes, beneficially owns the Notes, the tax treatment of the partnership and each partner in such partnership will

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generally depend upon the status of the partner and the activities of the partnership. Partnerships acquiring Notes, and partners in such partnerships, should consult their tax advisors as to the particular U.S. federal income tax considerations relevant to the acquisition, ownership and disposition of the Notes applicable to them.

Persons considering the purchase of Notes are urged to consult their tax advisors with respect to the U.S. federal income tax considerations relating to the acquisition, ownership and disposition of the Notes in light of their particular circumstances, as well as any tax consequences arising under the laws of any state, local or foreign taxing jurisdiction. Furthermore, this discussion does not describe the effect of U.S. federal estate and gift tax laws or the effect of any applicable foreign, state or local law.

Additional Payments

NiSource may redeem some or all of each series of the Notes at the redemption prices discussed under the "Supplemental Description of the Notes-Optional Redemption" and "-Tax Credit Event Redemption" sections of this prospectus supplement. Treasury regulations provide special rules for the treatment of debt instruments that provide for contingent payments. Under these regulations, a contingency is disregarded if the contingency is remote or incidental. Notwithstanding the possibility of additional payments in connection with an optional redemption of the Notes, NiSource intends to take the position that the Notes are not treated as "contingent payment debt instruments" for U.S. federal income tax purposes. This position is based in part on our belief that, as of the date of issuance of the Notes, the likelihood of making any such additional payments is remote. Our position is binding on a holder unless such holder discloses its contrary position in the manner required by the applicable Treasury regulations. The IRS, however, may take a contrary position, which could affect the timing and character of a holder's income. Holders are urged to consult their tax advisors regarding the potential application to the Notes of the contingent payment debt instrument rules and the consequences thereof. The remainder of this discussion assumes that the Notes are not treated as contingent payment debt instruments.

U.S. Holders

Taxation of Interest

All of the Notes bear interest at a fixed rate. We do not intend to issue the Notes at a discount that will equal or exceed a de minimis amount of original issue discount ("OID") and it is assumed for purposes of this discussion that the Notes will be issued with less than a de minimis amount of OID. Accordingly, payments of stated interest on the Notes generally will be taxable to a U.S. Holder as ordinary income at the time that such payments are received or accrued, in accordance with such U.S. Holder's method of tax accounting for U.S. federal income tax purposes.

Sale, Exchange, Redemption or other Taxable Disposition of the Notes

A U.S. Holder generally will recognize gain or loss on the sale, exchange, redemption or other taxable disposition of a Note equal to the difference between the amount realized upon the disposition (less a portion allocable to any accrued and unpaid interest, which will be taxable as interest as discussed above) and the U.S. Holder's adjusted tax basis in the Note. A U.S. Holder's adjusted tax basis in a Note generally will be equal to the amount that the U.S. Holder paid for the Note. Any gain or loss will be a capital gain or loss and will be a long-term capital gain or loss if, at the time of the sale, exchange redemption or other taxable disposition of the Note, the U.S. Holder has held the Note for more than one year; otherwise, the capital gain or loss will be short-term. Non-corporate U.S. Holders, including individuals, may be subject to a lower federal income tax rate on their net long-term capital gains than that applicable to ordinary income. All taxpayers are subject to certain limitations on the deductibility of their capital losses.

Information Reporting and Backup Withholding

Information returns will be filed annually with the IRS in connection with NiSource's payment of interest on the Notes. Information returns may also be filed with the IRS in connection with the proceeds from a sale or other

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disposition of the Notes, and the U.S. Holder may be subject to backup withholding tax (currently at a rate of 24%) on payments of interest on the Notes or on the proceeds from a sale or other disposition of the Notes. Amounts withheld under the backup withholding rules are not additional taxes. The amount of any backup withholding from a payment to a U.S. Holder may be allowed as a credit against the U.S. Holder's U.S. federal income tax liability or may entitle the U.S. Holder to a refund, provided that the required information is furnished to the IRS in a timely manner. U.S. Holders should consult their tax advisors as to their qualification for exemption from backup withholding and the procedure for obtaining such exemption.

Non-U.S. Holders

Taxation of Interest

Subject to the discussion under "-Information Reporting and Backup Withholding" and "-Foreign Account Tax Compliance Act Withholding" below, payments of interest on a Note to any Non-U.S. Holder generally will not be subject to U.S. federal income or withholding tax provided we or the person otherwise responsible for withholding U.S. federal income tax from payments on the Notes receives a required certification from the Non-U.S. Holder and the holder is not:

In order to satisfy the certification requirement, the Non-U.S. Holder must provide a properly completed IRS Form W-8BEN or Form W-8BEN-E, as appropriate (or substitute Form W-8BEN or IRS Form W-8BEN-E or the appropriate successor form of either) under penalties of perjury that provides the Non-U.S. Holder's name and address and certifies that the Non-U.S. Holder is not a U.S. Person. Alternatively, in the case where a security clearing organization, bank, or other financial institution holds the Notes in the ordinary course of its trade or business on behalf of the Non-U.S. Holder, certification requires that we or the person who otherwise would be required to withhold U.S. federal income tax receive from the financial institution a certification under penalties of perjury that a properly completed IRS Form W-8BEN or IRS Form W-8BEN-E, as appropriate (or substitute Form W-8BEN or Form W-8BEN-E or the appropriate successor form for either) has been received by it from the Non-U.S. Holder, and a copy of such form is furnished to us or the person who otherwise would be required to withhold U.S. federal income tax. In addition, we or our paying agent must not have actual knowledge or reason to know that the beneficial owner of the Notes is a U.S. Person.

A Non-U.S. Holder that does not qualify for exemption from withholding under the preceding paragraphs generally will be subject to withholding of U.S. federal income tax, currently at a 30% rate (or a lower rate under an applicable tax treaty, subject to meeting certification requirements similar to those set forth above), on payments of interest on the Notes that are not effectively connected with the conduct by the Non-U.S. Holder of a trade or business in the United States.

If the payments of interest on a Note are effectively connected with the conduct by a Non-U.S. Holder of a trade or business in the United States (and, if required by an applicable income tax treaty, are attributable to a permanent establishment or a fixed base maintained by the Non-U.S. Holder in the United States), such payments will be subject to U.S. federal income tax on a net basis at the rates applicable to U.S. Persons generally. If the

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Non-U.S. Holder is a corporation for U.S. federal income tax purposes, such payments also may be subject to a 30% branch profits tax (or a lower rate if so specified by an applicable income tax treaty). If payments are subject to U.S. federal income tax on a net basis in accordance with the rules described in the preceding two sentences, such payments will not be subject to U.S. withholding tax so long as the Non-U.S. Holder provides us, or the person who otherwise would be required to withhold U.S. federal income tax, with the appropriate certification.

In order to claim a tax treaty benefit or exemption from withholding with respect to income that is effectively connected with the conduct of a trade or business in the United States by a Non-U.S. Holder, the Non-U.S. Holder must provide a properly executed IRS Form W-8BEN, IRS Form W-8BEN-E or IRS Form W-8ECI (or a suitable substitute or successor form or such other form as the IRS may prescribe). Under Treasury regulations, a Non-U.S. Holder may under certain circumstances be required to obtain a U.S. taxpayer identification number and make certain certifications to us.

Non-U.S. Holders should consult their tax advisors regarding any applicable income tax treaties, which may provide for a lower rate of withholding tax, exemption from or reduction of branch profits tax or other rules different from those described above.

Sale, Exchange, Redemption or Other Taxable Disposition of the Notes

Subject to the discussion under "-Information Reporting and Backup Withholding" below, any gain realized by a Non-U.S. Holder on the sale, exchange, redemption or other taxable disposition of a Note generally will not be subject to U.S. federal income tax, unless:

A Non-U.S. Holder described in the first bullet point above will be required to pay U.S. federal income tax on the net gain derived from the sale, exchange, redemption or other taxable disposition generally in the same manner as if such Non-U.S. Holder were a U.S. Holder, and if such Non-U.S. Holder is a foreign corporation, it may also be required to pay an additional branch profits tax at a 30% rate (or a lower rate if so specified by an applicable income tax treaty). A Non-U.S. Holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or, if applicable, a lower treaty rate) on the gain derived from the sale, exchange, redemption or other taxable disposition, which may be offset by certain U.S. source capital losses, even though the Non-U.S. Holder is not considered a resident of the United States. Proceeds realized from the sale, exchange, redemption or other taxable disposition of a Note that are attributable to accrued but unpaid interest generally will be subject to, or exempt from, tax to the same extent as described above under "-Taxation of Interest" with respect to interest paid on a Note.

Information Reporting and Backup Withholding

Information returns will be filed annually with the IRS in connection with NiSource's payment of interest on the Notes. Copies of these information returns may also be made available under the provisions of a specific tax treaty or other agreement to the tax authorities of another country. Unless a Non-U.S. Holder complies with certification procedures to establish that it is not a U.S. Person, information returns may be filed with the IRS in connection with the proceeds from a sale or other disposition of the Notes, and the Non-U.S. Holder may be subject to backup withholding tax (currently at a rate of 24%) on payments of interest on the Notes or on the proceeds from a sale or other disposition of the Notes. The certification procedures required to claim the exemption from withholding tax on interest described above will satisfy the certification requirements necessary to avoid the backup withholding tax as well. Amounts withheld under the backup withholding rules are not

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additional taxes. The amount of any backup withholding from a payment to a Non-U.S. Holder may be allowed as a credit against the Non-U.S. Holder's U.S. federal income tax liability or may entitle the Non-U.S. Holder to a refund, provided that the required information is furnished to the IRS in a timely manner.

Foreign Account Tax Compliance Act Withholding

Under the Foreign Account Tax Compliance Act ("FATCA") and additional guidance issued by the IRS, a U.S. federal withholding tax of 30% generally will apply to interest on Notes paid to (i) a foreign financial institution (as a beneficial owner or as an intermediary), unless such institution enters into an agreement with the U.S. government to collect and provide to the U.S. tax authorities substantial information regarding U.S. account holders of such institution (which would include certain equity and debt holders of such institution, as well as certain account holders that are foreign entities with U.S. owners), or (ii) a foreign entity that is not a financial institution (as a beneficial owner or as an intermediary), unless such entity provides the withholding agent with a certification identifying the substantial U.S. owners of the entity, which generally includes any U.S. Person who directly or indirectly owns more than 10% of the entity. NiSource will not pay any additional amounts to "gross up" payments to holders as a result of any withholding or deduction for such taxes. Non-U.S. Holders are encouraged to consult with their tax advisors regarding the possible implications of the FATCA withholding rules on their investment in the Notes.

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UNDERWRITING (CONFLICTS OF INTEREST)

Subject to the conditions set forth in the underwriting agreement, we have agreed to sell all, but not less than all, of the Notes to the underwriters listed below, for whom BNP Paribas Securities Corp., BofA Securities, Inc., MUFG Securities Americas Inc. and U.S. Bancorp Investments, Inc. are acting as representatives, and the underwriters have severally and not jointly agreed to purchase the principal amount of each series of the Notes set forth opposite its name in the following table:

The underwriting agreement provides that the underwriters are obligated to purchase all of the Notes of a series if any Notes of such series are purchased. The underwriting agreement also provides that if an underwriter defaults, the purchase commitments of non-defaulting underwriters may be increased or this offering of the Notes of such series may be terminated.

The underwriters propose to offer the Notes initially at the prices to public set forth on the cover page of this prospectus supplement and to certain dealers at that price less a selling concession of     % of the principal amount per 2031 Note and    % of the principal amount per 2036 Note. The underwriters may allow, and those certain dealers may reallow, a discount of     % of the principal amount per 2031 Note and     % of the principal amount per 2036 Note on sales to certain other dealers. After the initial public offering of the Notes, the prices to public and other selling terms may be changed.

We estimate that our total expenses for this offering, excluding the underwriting discounts, will be approximately $    .

Each series of Notes will be a new issuance of securities with no established trading market. One or more of the underwriters intends to make a secondary market for the Notes. However, they are not obligated to do so and may discontinue making a secondary market for the Notes at any time without notice. No assurance can be given as to how liquid the trading market for the Notes will be.

We have agreed to indemnify the underwriters against liabilities under the Securities Act or contribute to payments which the underwriters may be required to make in that respect.

This offering of the Notes by the underwriters is subject to receipt and acceptance and subject to the underwriters' right to reject any order in whole or part.

In connection with this offering, the underwriters may engage in stabilizing transactions, over-allotment transactions, syndicate covering transactions and penalty bids in accordance with Regulation M under the Exchange Act.

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These stabilizing transactions, syndicate-covering transactions and penalty bids may have the effect of raising or maintaining the market price of the Notes or preventing or retarding a decline in the market prices of the Notes. As a result, the price of the Notes may be higher than the prices that might otherwise exist in the open market. The underwriters are not required to engage in these transactions and these transactions, if commenced, may be discontinued at any time.

We expect to deliver the Notes against payment for the Notes on or about the date specified in the last paragraph on the cover page of this prospectus supplement, which will be the     business day following the date of the pricing of the Notes. Under Rule 15c6-1 under the Exchange Act, trades in the secondary market generally are required to settle in one business day, unless the parties to a trade expressly agree otherwise. Accordingly, purchasers who wish to trade Notes on any date prior to the first business day before delivery of the Notes will be required, by virtue of the fact that the Notes initially will settle in T+    , to specify alternative settlement arrangements to prevent a failed settlement. Purchasers of the Notes who wish to trade the Notes on any date prior to the first business day before delivery of the Notes should consult their own advisors.

Other Relationships

Certain of the underwriters and their affiliates have provided certain investment banking, commercial banking and other financial services to us and our affiliates, for which they have received customary fees. The underwriters and their affiliates may from time to time engage in future transactions with us and our affiliates and provide services to us and our affiliates in the ordinary course of their business.

In the ordinary course of their various business activities, the underwriters and their respective affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own accounts and for the accounts of their customers, and such investments and securities activities may involve our securities and/or instruments. Certain of the underwriters or their affiliates have a lending relationship with us. Certain of those underwriters or their affiliates routinely hedge, and certain other of those underwriters or their affiliates may hedge, their credit exposure to us consistent with their customary risk management policies. Typically, such underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the Notes offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the Notes offered hereby. The underwriters and their respective affiliates may also make investment recommendations and/or publish or express independent research views in respect to such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Conflicts of Interest

Affiliates of certain of the underwriters are lenders under our revolving credit facility. To the extent that we use the aggregate net proceeds from this offering to repay amounts we have borrowed or may borrow or

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re-borrow in the future under our revolving credit facility, these lenders will receive their pro rata portions of such proceeds. Further, certain of the underwriters are dealers under our commercial paper program. To the extent we use the aggregate net proceeds from this offering to repay notes issued under our commercial paper program that are held by one or more underwriters, such underwriters will receive proceeds from this offering.

Because some of the aggregate net proceeds of this offering may be used to repay amounts outstanding under our revolving credit facility or to repay notes issued under our commercial paper program that are held by certain underwriters, such underwriters would be deemed to have a conflict of interest under FINRA Rule 5121 to the extent such underwriters or affiliates receive at least 5% of the aggregate net proceeds of this offering. Any underwriter deemed to have a conflict of interest would be required to conduct the distribution of the Notes in accordance with FINRA Rule 5121. If this offering is conducted in accordance with FINRA Rule 5121, such underwriter would not be permitted to confirm a sale to an account over which it exercises discretionary authority without first receiving specific written approval from the account holder. See the "Use of Proceeds" section of this prospectus supplement for more information.

Selling Restrictions

European Economic Area

The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area ("EEA"). For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of Directive 2014/65/EU (as amended, "MiFID II"); or (ii) a customer within the meaning of Directive 2016/97/EU where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in Regulation 2017/1129/EU (as amended, the "Prospectus Regulation"). No key information document required by Regulation (EU) No 1286/2014 (as amended, the "PRIIPs Regulation") for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared. Offering or selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPs Regulation. This prospectus supplement and the accompanying prospectus have been prepared on the basis that any offer of Notes in any Member State of the EEA will be made pursuant to an exemption under the Prospectus Regulation from the requirement to publish a prospectus. This prospectus supplement and the accompanying prospectus are not prospectuses for the purposes of the Prospectus Regulation.

For the purposes of this provision, the expression an "offer to the public" in relation to any Notes in any Member State of the EEA means the communication in any form and by any means of sufficient information on the terms of the offer and any Notes to be offered so as to enable an investor to decide to purchase any Notes.

United Kingdom

The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the United Kingdom (the "UK"). For these purposes, a retail investor means a person who is neither: (i) a professional client, as defined in point (8) of Article 2(1) of Regulation (EU) No 600/2014 as it forms part of domestic law by virtue of the European Union (Withdrawal) Act 2018 (the "EUWA"); nor (ii) a qualified investor as defined in paragraph 15 of Schedule 1 to the Public Offers and Admissions to Trading Regulations 2024 (the "POATRs"). Consequently, no key information document required by Regulation (EU) No 1286/2014 as it forms part of domestic law by virtue of the EUWA (the "UK PRIIPs Regulation") for offering or selling the Notes or otherwise making them available to retail investors in the UK has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the UK may be unlawful under the UK PRIIPs Regulation.

This prospectus supplement and the accompanying prospectus have been prepared on the basis that this offering falls within one of the exceptions specified in Part 1 of Schedule 1 of the POATRs. Accordingly, there

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will not be a prospectus prepared or published for the purposes of the POATRs. Neither this prospectus supplement nor the accompanying prospectus is a prospectus for the purposes of the POATRs or the Financial Services and Markets Act 2000 (the "FSMA"), and any offer of the Notes in the UK is made pursuant to an exemption under the POATRs or the FSMA.

This prospectus supplement and the accompanying prospectus have not been approved by an authorized person for the purposes of section 21 of the FSMA and accordingly, are only being distributed to, and must not be passed on to, the general public in the UK. In the UK, such documents are only directed at, persons who are (i) investment professionals falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (the "Order"), (ii) high net worth entities or other persons falling within Article 49(2)(a) to (d) of the Order, or (iii) other persons to whom they may otherwise lawfully be communicated (each such person being referred to as a "relevant person"). This prospectus supplement, the accompanying prospectus and their contents are confidential and should not be distributed, published or reproduced (in whole or in part) or disclosed by recipients to any other persons in the United Kingdom. Any investment or investment activity to which this prospectus supplement and the accompanying prospectus relate is available only to and will be engaged in only with relevant persons. Any person in the UK who is not a relevant person should not act or rely on this prospectus supplement and the accompanying prospectus or any of their contents.

Switzerland

This prospectus supplement and the accompanying prospectus are not intended to constitute an offer or solicitation to purchase or invest in the Notes. The Notes may not be publicly offered, directly or indirectly, in Switzerland within the meaning of the Swiss Financial Services Act ("FinSA") and no application has or will be made to admit the Notes to trading on any trading venue (exchange or multilateral trading facility) in Switzerland. Neither this prospectus supplement, the accompanying prospectus nor any other offering or marketing material relating to the Notes constitutes a prospectus pursuant to the FinSA, and neither this prospectus supplement, the accompanying prospectus nor any other offering or marketing material relating to the Notes may be publicly distributed or otherwise made publicly available in Switzerland.

Canada

The Notes may be sold in Canada only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the Notes must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement and the accompanying prospectus (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser's province or territory.

The purchaser should refer to any applicable provisions of the securities legislation of the purchaser's province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts ("NI 33-105"), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

Japan

This offering of the Notes has not been and will not be registered under the Financial Instruments and Exchange Act of Japan (Act No. 25 of 1948, as amended; the "FIEA"). Accordingly, the Notes may not be

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offered or sold, directly or indirectly, in Japan or to, or for the account or benefit of, any "resident" of Japan (which term as used herein means any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to, or for the account or benefit of, others for re-offering or resale, directly or indirectly, in Japan or to, or for the account or benefit of, any resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the FIEA and any other applicable laws, regulations and ministerial guidelines of Japan in effect at the relevant time.

Republic of Korea

The Notes may not be offered, sold and delivered directly or indirectly, or offered or sold to any person for reoffering or resale, directly or indirectly, in the Republic of Korea or to any resident of the Republic of Korea except pursuant to the applicable laws and regulations of the Republic of Korea, including, without limitation, the Financial Investment Services and Capital Markets Act and the Foreign Exchange Transaction Law and the decrees and regulations thereunder. The Notes have not been and will not be registered with the Financial Services Commission of Korea for public offering in the Republic of Korea. Furthermore, the Notes may not be resold to residents of the Republic of Korea unless the purchaser of the Notes complies with all applicable regulatory requirements (including but not limited to government approval requirements under the Foreign Exchange Transaction Law and its subordinate decrees and regulations) in connection with the purchase of the Notes.

Singapore

This prospectus supplement and the accompanying prospectus have not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus supplement and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of Notes may not be circulated or distributed, nor may the Notes be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor as defined in Section 4A of the Securities and Futures Act, 2001 of Singapore, as modified or amended from time to time (the "SFA"), pursuant to Section 274 of the SFA, (ii) to a relevant person (as defined in Section 275(2) of the SFA) pursuant to Section 275(1) of the SFA, or any person pursuant to Section 275(1A) of the SFA, and in accordance with the conditions specified in Section 275, of the SFA and (where applicable) Regulation 3 of the Securities and Futures (Classes of Investors) Regulations 2018 of Singapore, or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

Where the Notes are subscribed or purchased pursuant to an offer made in reliance on Section 275 of the SFA by a relevant person which is:

securities or securities based derivative contracts (each term as defined in Section 2(1) of the SFA) of that corporation or the beneficiaries' rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the Notes pursuant to an offer made under Section 275 of the SFA except:

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Singapore SFA Product Classification-Solely for the purposes of our obligations pursuant to section 309B of the SFA and the Securities and Futures (Capital Markets Products) Regulations 2018 of Singapore (the "CMP Regulations 2018"), we have determined, and hereby notify all relevant persons (as defined in Section 309A(1) of the SFA) that the Notes are "prescribed capital markets products" (as defined in the CMP Regulations 2018) and Excluded Investment Products (as defined in MAS Notice SFA 04-N12: Notice on the Sale of Investment Products and MAS Notice FAA-N16: Notice on Recommendations on Investment Products).

Taiwan

The Notes have not been and will not be registered or filed with, or approved by, the Financial Supervisory Commission of Taiwan, Republic of China ("Taiwan") and/or any other regulatory authority of Taiwan pursuant to relevant securities laws and regulations and may not be offered, issued or sold within Taiwan through a public offering or in any manner which would constitute an offer within the meaning of the Securities and Exchange Act of Taiwan or relevant laws and regulations that requires a registration, filing or approval of the Financial Supervisory Commission of Taiwan and/or other regulatory authority of Taiwan. No person or entity in Taiwan has been authorized to offer, sell, distribute or otherwise intermediate this offering of the Notes in Taiwan.

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LEGAL MATTERS

The validity of the Notes will be passed upon for us by McGuireWoods LLP, Charlotte, North Carolina. The underwriters have been represented by Hunton Andrews Kurth LLP, New York, New York.

EXPERTS

The consolidated financial statements, and the related financial statement schedule, incorporated by reference in this prospectus supplement from the NiSource Inc. Annual Report on Form 10-K for the year ended December 31, 2025, and the effectiveness of NiSource Inc. and subsidiaries' internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports. Such consolidated financial statements and financial statement schedule are incorporated by reference in reliance upon the reports of such firm given their authority as experts in accounting and auditing.

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PROSPECTUS

NiSource Inc.

Common Stock

Preferred Stock

Depositary Shares

Debt Securities

Warrants

Stock Purchase Contracts

Stock Purchase Units

NiSource Inc. may offer, from time to time, in amounts, at prices and on terms that it will determine at the time of offering, any or all of the following:

We will provide specific terms of these securities, including their offering prices, in prospectus supplements to this prospectus. The prospectus supplements may also supplement, update or amend information contained in this prospectus. You should read this prospectus and any prospectus supplement carefully before you invest.

We may offer these securities to or through underwriters, through dealers or agents, directly to you or through a combination of these methods. You can find additional information about our plan of distribution for the securities under the heading "Plan of Distribution" beginning on page 29 of this prospectus. We will also describe the plan of distribution for any particular offering of these securities in the applicable prospectus supplement. This prospectus may not be used to sell our securities unless it is accompanied by a prospectus supplement.

Our common stock is listed on the New York Stock Exchange under the symbol "NI."

Investing in our securities involves risks. You should carefully consider the risk factors described under the heading "Risk Factors" on page 5 of this prospectus, in the documents that are incorporated by reference into this prospectus and, if applicable, in risk factors described in any accompanying prospectus supplement before you invest in our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is October 30, 2025.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we have filed with the Securities and Exchange Commission ("SEC"), utilizing a "shelf" registration process. Under this process, we may from time to time sell any combination of the securities described in this prospectus in one or more offerings.

This prospectus provides you with a general description of the common stock, preferred stock, depositary shares, debt securities, warrants, stock purchase contracts and stock purchase units we may offer. Each time we offer securities, we will provide a prospectus supplement that will contain the specific terms of these securities and the specific manner in which these securities will be offered and sold. That prospectus supplement may include a description of any risk factors or other special considerations applicable to those securities. The prospectus supplement may also supplement, update or amend information contained in this prospectus. If there is a conflict or any inconsistency between the information contained in this prospectus, on the one hand, and the information contained in any prospectus supplement, on the other hand, you should rely on the information in the prospectus supplement.

The registration statement containing this prospectus, including the exhibits to the registration statement, provides additional information about us and the securities offered under this prospectus. You should read the exhibits carefully for provisions that may be important to you. Specifically, we have filed and incorporated by reference certain legal documents that control the terms of the securities offered by this prospectus as exhibits to the registration statement. We will file or incorporate by reference certain other legal documents that will control the terms of the securities we may offer by this prospectus as exhibits to the registration statement or to reports we file with the SEC that are incorporated by reference into this prospectus.

In addition, we may prepare and deliver one or more "free writing prospectuses" to you in connection with any offering of securities under this prospectus. Any such free writing prospectus may contain additional information about us, our business, the offered securities, the manner in which such securities are being offered, our intended use of the proceeds from the sale of such securities, risks relating to our business or an investment in such securities or other information.

This prospectus and certain of the documents incorporated by reference into this prospectus contain, and any accompanying prospectus supplement or free writing prospectus that we deliver to you may contain, summaries of information contained in documents that we have filed or will file as exhibits to our SEC filings. Such summaries do not purport to be complete and are subject to, and qualified in their entirety by reference to, the actual documents filed with the SEC.

You should carefully read this entire prospectus and the accompanying prospectus supplement, if applicable, including the documents incorporated by reference as described under the heading "Incorporation by Reference," and any related free writing prospectuses before deciding whether to invest in any securities offered by this prospectus and any accompanying prospectus supplement. We have not authorized anyone to provide you with different or additional information, other than the information described in the preceding sentence. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give to you.

The information in this prospectus, any accompanying prospectus supplement and the documents incorporated by reference herein and therein is accurate only as of the date of the respective documents in which the information appears. Our business, financial condition, results of operations and prospects may have changed since those dates, and neither the delivery of this prospectus and any accompanying prospectus supplement nor any sale hereunder shall, under any circumstances, create any implication to the contrary.

We have not taken any action that would permit the offering or possession or distribution of this prospectus or any applicable prospectus supplement in any jurisdiction where action for that purpose is required, other than

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in the United States, and we are not making an offer to sell or soliciting an offer to buy these securities in any jurisdiction in which the offer or solicitation is not authorized or in which the person making the offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make the offer or solicitation. Persons outside the United States who come into possession of this prospectus or any applicable prospectus supplement must inform themselves about, and observe any restrictions relating to, the offering of the securities described herein and the distribution of this prospectus or any applicable prospectus supplement outside the United States.

When we refer to "NiSource," "we," "our," "ours" and "us" in this prospectus under the heading "Forward-Looking Statements" we mean NiSource Inc. and its subsidiaries, through which substantially all of NiSource Inc.'s operations are conducted. When such terms are used elsewhere in this prospectus, we refer only to NiSource Inc., as the issuer of securities registered hereunder, and not to any of its direct or indirect subsidiaries or affiliates except as expressly provided or the context otherwise requires. Unless the context requires otherwise, references to "securities" refer collectively to the common stock, preferred stock, depositary shares, debt securities, warrants, stock purchase contracts and stock purchase units registered hereunder.

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NISOURCE INC.

Overview. NiSource is an energy holding company whose primary subsidiaries are fully regulated natural gas and electric utility companies, serving approximately 3.8 million customers in six states. We derive substantially all of our operating income through these rate-regulated businesses, which are summarized for financial reporting purposes into two primary reportable segments:

Business Strategy. Our business strategy focuses on providing safe and reliable service through our core, rate-regulated, asset-based utilities, with the goal of adding value to all of our stakeholders. Our utilities continue to advance our core safety, infrastructure and environmental investment programs supported by complementary regulatory and customer initiatives across the six states in which we operate. Our goal is to develop strategies that (i) support long-term infrastructure investment and safety programs to better serve our customers, (ii) align our tariff structures with our cost structure, and (iii) drive value and enable growth in an evolving energy ecosystem. These strategies focus on improving safety and reliability, enhancing customer experience, pursuing regulatory and legislative initiatives to increase accessibility for customers currently not on our gas and electric service, ensuring customer affordability and reducing emissions while generating sustainable returns.

In addition to these core operations, we are serving increasing load growth from data center customers in our northern Indiana service territory. We believe data center development can enhance our local tax base, diversify the employment base across the state of Indiana, and provide greater value to existing customers and shareholders. We continually evaluate ways to effectively manage the potential power demand, generation sources, and transmission capabilities to meet potential further load growth from additional data center customers, while at the same time focusing on the community, financial, operational and regulatory factors that must be managed effectively in order to succeed with our data center strategy, as well as our environmental goals.

Columbia Operations. Columbia Operations aggregates the results of the fully regulated and wholly-owned subsidiaries of NiSource Gas Distribution Group, Inc. (a holding company that owns Columbia Gas of Kentucky, Inc., Columbia Gas of Maryland, Inc., Columbia Gas of Ohio, Inc., Columbia Gas of Pennsylvania, Inc., and Columbia Gas of Virginia, Inc.). Each Columbia distribution company is an operating segment which we aggregate to form the Columbia Operations reportable segment. Through our wholly-owned subsidiary NiSource Gas Distribution Group, Inc., we provide natural gas to approximately 2.4 million residential, commercial and industrial customers in Ohio, Pennsylvania, Virginia, Kentucky, and Maryland. We operate approximately 37,200 miles of distribution main pipeline plus the associated individual customer service lines and 330 miles of transmission main pipeline located in our service areas. There were no significant disruptions to our system or facilities during 2024.

NIPSCO Operations. NIPSCO Operations aggregates the results of NIPSCO Holdings I, and its majority- owned subsidiaries, including Northern Indiana Public Service Company LLC ("NIPSCO"), in which we own an 80.1% indirect interest and which has both fully regulated gas and electric operations in northern Indiana. We distribute natural gas to approximately 0.9 million customers in northern Indiana through our subsidiary NIPSCO. We also generate, transmit and distribute electricity through our subsidiary NIPSCO to approximately 0.5 million customers in 20 counties in the northern part of Indiana and also engage in wholesale electric and transmission transactions. We own and operate generation assets as well as source power through power purchase agreements ("PPAs"). We continue to transition our generation portfolio from coal-fired generation to lower-emission sources. We currently have eight owned renewable generation facilities in service: Rosewater Wind Generation LLC ("Rosewater"), Indiana Crossroads Wind Generation LLC ("Indiana Crossroads Wind"), Indiana Crossroads Solar Generation LLC ("Indiana Crossroads Solar"), Dunns Bridge I Solar Generation LLC ("Dunns Bridge I"), Cavalry Solar Generation Center ("Cavalry"), Dunns Bridge II Solar Generation Center ("Dunns

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Bridge II"), Fairbanks Solar Generation LLC ("Fairbanks") and Gibson Solar LLC ("Gibson"). Rosewater went into service in December 2020 and Indiana Crossroads Wind went into service in December 2021. Indiana Crossroads Solar and Dunns Bridge I went into service in June 2023. Cavalry went into service May 2024, Dunns Bridge II went into service in January 2025, Fairbanks went into service in May 2025 and Gibson went into service August 2025. As of December 31, 2024, we had multiple PPAs that provide 600 megawatts of capacity, with contracts expiring between 2038 and 2040. NIPSCO's transmission system, with voltages from 69,000 to 765,000 volts, consists of approximately 3,000 circuit miles. NIPSCO is interconnected with eight neighboring electric utilities. We operate 66 transmission and 250 distribution substations, and own approximately 311,300 poles. Our generating facilities had no material unplanned interruptions during 2024.

NIPSCO participates in the Midcontinent Independent System Operator ("MISO") transmission service and wholesale energy market. NIPSCO has transferred functional control of its electric transmission assets to MISO, and transmission service for NIPSCO occurs under the MISO Open Access Transmission Tariff. NIPSCO generating units are dispatched by MISO which takes into account economics, reliability of the MISO system and unit availability. During the year ended December 31, 2024, NIPSCO generating units, inclusive of its owned renewable generation facilities, were dispatched to meet 55.4% of its overall system load, and the remainder of the overall system load was procured through PPAs and the MISO market.

During the quarter ended September 30, 2025, NIPSCO entered into an agreement to provide electric service to a large, investment grade data center customer (the "data center contract"), NIPSCO's first such agreement with a data center customer. NIPSCO continues to experience strong demand from potential data center customers in its northern Indiana service territory and continues to evaluate potential future data center opportunities. In order to provide electric services to its data center customers, NIPSCO plans to enter into PPAs with NIPSCO Generation LLC ("GenCo"), an affiliate of NIPSCO in which we own an 80.1% indirect interest, and GenCo is expected to construct the generation assets necessary to fulfill data center customer demand. GenCo's operations in connection these PPAs and serving data center customers will be regulated by the IURC pursuant to GenCo's declination of jurisdiction petition with IURC, which was approved on September 24, 2025, including submission of the data center contract and future data center contracts to the IURC for approval.

Our principal executive offices are located at 801 East 86th Avenue, Merrillville, Indiana 46410, telephone: (877) 647-5990.

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RISK FACTORS

Investing in the securities involves risk. You should read carefully the "Risk Factors" and "Note regarding forward-looking statements" sections in NiSource's most recent Annual Report on Form 10-K and in NiSource's subsequent Quarterly Reports on Form 10-Q, which are incorporated by reference in this prospectus. Before making an investment decision, you should carefully consider these risks as well as other information contained or incorporated by reference in this prospectus, and corresponding sections in reports NiSource may file with the SEC after the date of this prospectus. The prospectus supplement applicable to each type or series of securities we offer may contain a discussion of additional risks applicable to an investment in us and the particular type of securities we are offering under that prospectus supplement.

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FORWARD-LOOKING STATEMENTS

Some of the information included in this prospectus, in any prospectus supplement and in the documents incorporated by reference are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended (the "Securities Act"), and Section 21E of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). These forward-looking statements include, but are not limited to, statements concerning our plans, strategies, objectives, expected performance, expenditures, recovery of expenditures through rates, stated on either a consolidated or segment basis, and any and all underlying assumptions and other statements that are other than statements of historical fact. Expressions of future goals and expectations and similar expressions, including "may," "will," "should," "could," "would," "aims," "seeks," "expects," "plans," "projects," "anticipates," "intends," "believes," "estimates," "predicts," "potential," "targets," "forecast," "guidance," "outlook," and "continue," reflecting something other than historical fact are intended to identify forward-looking statements. All forward-looking statements are based on assumptions that management believes to be reasonable; however, there can be no assurance that actual results will not differ materially. Various factors may cause actual results to be materially different than the suggested outcomes within forward-looking statements; accordingly, there is no assurance that such results will be realized. These factors include, but are not limited to:

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For more information about the significant risks that could affect the outcome of these forward-looking statements and our future financial condition, results of operations, liquidity and cash flows, you should read the sections titled "Risk Factors" in the documents incorporated by reference in this prospectus and any accompanying prospectus supplement, together with "Risk Factors" in this prospectus. In light of these risks, uncertainties and assumptions, the events described in the forward-looking statements included or incorporated by reference in this prospectus and any accompanying prospectus supplement might not occur or might occur to a different extent or at a different time than described. We qualify all of our forward-looking statements by these cautionary statements. Forward-looking statements speak only as of the date they are made, and we expressly disclaim an obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, except as may be required by law.

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WHERE YOU CAN FIND MORE INFORMATION

We are subject to the informational requirements of the Exchange Act, and, in accordance therewith, file annual reports on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, proxy and information statements, and amendments to reports filed or furnished pursuant to Sections 13(a), 14 and 15(d) of the Exchange Act with the SEC. Our filings with the SEC, as well as additional information about us, are available to the public through our website at http://www.nisource.com and are made available as soon as reasonably practicable after such material is filed with or furnished to the SEC. The information contained on our or accessible through our website is not incorporated by reference herein and is not part of this prospectus or the registration statement of which this prospectus is a part. The SEC maintains a website at www.sec.gov that contains reports, proxy and information statements, and other information regarding NiSource and other companies that file materials electronically with the SEC.

INCORPORATION BY REFERENCE

The SEC allows us to "incorporate by reference" information into this prospectus. This means that we can disclose important business, financial and other information to you by referring you to another document that NiSource has filed separately with the SEC. The information incorporated by reference is considered to be part of this prospectus. Information in a document incorporated or deemed to be incorporated by reference in this prospectus that is filed after the date of this prospectus will automatically modify and supersede the information included or incorporated by reference in this prospectus to the extent that the subsequently filed information modifies or supersedes the existing information. We incorporate by reference the following documents filed with the SEC:

We will provide, without charge, to each person, including any beneficial owner, to whom this prospectus is delivered a copy of any of these filings at no cost upon written or oral request to us at the following address and telephone number: Corporate Secretary, NiSource Inc., 801 East 86th Avenue, Merrillville, Indiana 46410, telephone: (877) 647-5990.

We have filed this prospectus with the SEC as part of a registration statement on Form S-3 under the Securities Act. This prospectus does not contain all of the information included in the registration statement. Any statement made in this prospectus concerning the contents of any contract, agreement or other document is only a summary

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of the actual document. If we have filed or incorporated by reference any contract, agreement or other document as an exhibit to the registration statement, you should read the exhibit for a more complete understanding of the document or matter involved. Each statement regarding a contract, agreement or other document is qualified in its entirety by reference to the actual document.

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USE OF PROCEEDS

Unless otherwise described in the applicable prospectus supplement, we will use the net proceeds from the sale of securities offered by this prospectus and any applicable prospectus supplement for general corporate purposes, including additions to working capital and repayment of existing indebtedness. Accordingly, we will have significant discretion in the use of any such net proceeds. We may provide additional information on the use of the net proceeds from the issuance and sale of our securities in an applicable prospectus supplement relating to such securities.

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DESCRIPTION OF CAPITAL STOCK

General

The authorized capital stock of NiSource consists of 770,000,000 shares, of which 750,000,000 are common stock, par value $0.01, and 20,000,000 are preferred stock, par value $0.01. As of October 22, 2025, NiSource had outstanding 477,195,529 shares of its common stock and no shares of its preferred stock.

The below summaries of provisions of NiSource's common stock and preferred stock are not necessarily complete. You are urged to read carefully, and the below summaries are qualified in their entirety by, NiSource's certificate of incorporation and Amended and Restated By-Laws ("bylaws") which are filed as exhibits to the registration statement of which this prospectus is a part.

Anti-Takeover Provisions

NiSource's certificate of incorporation includes provisions that may have the effect of deterring hostile takeovers or delaying or preventing changes in control of NiSource's management. More specifically, the certificate of incorporation provides that stockholders may not cumulate their votes and stockholder action may be taken only at a duly called meeting and not by written consent. In addition, NiSource's bylaws contain requirements for advance notice of stockholder proposals and director nominations. These and other provisions of the certificate of incorporation and bylaws and Delaware law could discourage potential acquisition proposals for NiSource and could delay or prevent a change in control of NiSource.

Under Section 242(b)(2) of the Delaware General Corporation Law ("DGCL"), the approval of the holders of a majority of the outstanding shares of a class of NiSource's capital stock would be necessary to authorize any amendment to the certificate of incorporation that would increase or decrease the aggregate number of authorized shares of such class of capital stock or that would adversely alter or change the powers, preferences or special right of such class of capital stock. The effect of this provision may permit the holders of NiSource's outstanding shares of capital stock to block a proposed amendment to the certificate of incorporation in connection with a potential acquisition of NiSource if such amendment would adversely affect the powers, preferences or special rights of NiSource's capital stock.

NiSource is subject to the provisions of Section 203 of the DGCL regulating corporate takeovers. Section 203 prevents certain Delaware corporations, including those whose securities are listed on a national securities exchange, such as the New York Stock Exchange, from engaging, under certain circumstances, in a "business combination" (as defined therein), which includes, among other things, a merger or sale of more than 10% of the corporation's assets, with any interested stockholder for three years following the date that the stockholder became an interested stockholder. An interested stockholder is a stockholder who acquired 15% or more of the corporation's outstanding voting stock or an affiliate or associate of such person.

Common Stock

NiSource's common stock is listed on the New York Stock Exchange under the symbol "NI." Shares of NiSource's common stock, offered and sold pursuant to the registration statement of which this prospectus forms a part, will be fully paid and non-assessable.

Liquidation Rights

In the event of any liquidation, dissolution or winding up of NiSource, whether voluntary or involuntary, after payment or provision for payment of the debts and other liabilities of NiSource and the distribution in full of all preferential amounts, if any, to which the holders of any series of preferred stock of NiSource hereafter created are entitled, the holders of common stock will share ratably in the remaining assets in proportion to the number of shares of common stock held by them respectively. A consolidation or merger of NiSource with or into any

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other corporation, or any purchase or redemption of shares of any class of NiSource's capital stock, will not be deemed to be a liquidation, dissolution or winding up of NiSource's affairs.

Voting Rights

Except as otherwise required by Delaware law or as otherwise provided in the certificate of designations for any series of preferred stock of NiSource hereafter created, holders of NiSource's common stock exclusively possess voting power for the election of NiSource's directors and all other matters requiring stockholder action. Each holder of common stock, if entitled to vote on a matter, is entitled to one vote per share. Holders of common stock are not entitled to cumulative voting rights. Holders of common stock will be notified of any stockholders' meeting according to applicable law.

Dividend Rights

Holders of common stock will be entitled to receive dividends, when, as and if declared by NiSource's board of directors out of legally available funds for such purpose in accordance with Delaware law, subject to the restrictions described in the following paragraph and, if applicable, to the powers, preferences and rights afforded to the holders of any series of preferred stock of NiSource hereafter created. Dividends may be paid in cash, capital stock or other property of NiSource.

Under the terms of its outstanding junior subordinated notes, NiSource has the right, from time to time, to defer the interest payments on its outstanding junior subordinated notes on one or more occasions for up to ten consecutive years. In addition, NiSource may issue, from time to time, additional junior subordinated notes or other securities that (i) provide them with rights to defer the payment of interest or other payments and (ii) contain dividend restrictions in the event of the exercise of such rights. In the event that NiSource were to exercise any right to defer interest or other payments on currently outstanding or future series of junior subordinated notes or other securities, NiSource would be prohibited from declaring or paying any dividends or distributions on any shares of NiSource's common stock (other than dividends payable solely in shares of its common stock and certain other limited exceptions) or redeeming, repurchasing, acquiring or making a liquidation payment with respect to shares of its common stock during the periods in which such payments were deferred.

As noted above, NiSource is an energy holding company that derives substantially all of its revenues and earnings from the operating results of the rate-regulated businesses of its subsidiaries. Accordingly, NiSource's ability to pay dividends on its capital stock is dependent primarily upon the earnings and cash flows of its subsidiaries and the distribution or other payment of such earnings to NiSource. NiSource's subsidiaries are separate and distinct legal entities and have no obligation, contingent or otherwise, to pay any amounts on the capital stock of NiSource or to make any funds available therefor, whether by dividends, loans or other payments.

No Preemptive Rights

Holders of NiSource's common stock are not entitled to, as holders of common stock, any preemptive rights with respect to any shares of NiSource's capital stock or any of its securities convertible into or exercisable for its capital stock.

Preferred Stock

General

The board of directors of NiSource can, without approval of stockholders, issue one or more series of preferred stock. The board of directors of NiSource can also determine the rights, preferences and limitations of each series, including any dividend rights, voting rights, conversion rights, redemption rights and liquidation preferences, the number of shares constituting each series and the terms and conditions of issue. In some cases,

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the issuance of preferred stock could delay a change in control of NiSource and make it harder to remove incumbent management. All preferred stock will be fully paid and non-assessable.

In the event that NiSource were to exercise any right to defer interest or other payments on currently outstanding or future series of junior subordinated notes or other securities, as described under "Common Stock-Dividend Rights" above, NiSource would be prohibited from declaring or paying any dividends or distributions on any shares of NiSource's preferred stock (other than dividends payable solely in shares of its common stock and certain other limited exceptions) or redeeming, repurchasing, acquiring or making a liquidation payment with respect to shares of its preferred stock during the periods in which such payments were deferred.

In addition, under certain circumstances, preferred stock could also restrict dividend payments to holders of common stock.

The terms of the preferred stock that NiSource may offer will be established by or pursuant to a resolution of the board of directors of NiSource and will be issued under certificates of designations or through amendments to the certificate of incorporation. If NiSource uses this prospectus to offer preferred stock, an accompanying prospectus supplement will describe the specific terms of the preferred stock. NiSource will also indicate in the prospectus supplement whether the general terms and provisions described in this prospectus apply to the preferred stock that NiSource may offer. If there are differences between the prospectus supplement relating to a particular series and this prospectus, the prospectus supplement will control.

The following terms of the preferred stock, as applicable, will be set forth in a prospectus supplement relating to the preferred stock:

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The terms, if any, on which the preferred stock may be exchanged for or converted into shares of common stock or any other security and, if applicable, the conversion or exchange price, or how it will be calculated, and the conversion or exchange period will be set forth in the applicable prospectus supplement.

The preferred stock or any series of preferred stock may be represented, in whole or in part, by one or more global certificates, which will have an aggregate liquidation preference equal to that of the preferred stock represented by the global certificate.

Each global certificate will:

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DESCRIPTION OF DEPOSITARY SHARES

NiSource may issue depositary shares representing fractional interests in shares of our preferred stock of any series. The following description sets forth certain general terms and provisions of the depositary shares to which any prospectus supplement may relate. The particular terms of the depositary shares to which any prospectus supplement may relate and the extent, if any, to which the general terms and provisions may apply to the depositary shares so offered will be described in the applicable prospectus supplement. To the extent that any particular terms of the depositary shares, deposit agreements and depositary receipts described in a prospectus supplement differ from any of the terms described below, then the terms described below will be deemed to have been superseded by that prospectus supplement. You should read the applicable deposit agreement and depositary receipts for additional information before you decide whether to purchase any of NiSource's depositary shares.

In connection with the issuance of any depositary shares, NiSource will enter into a deposit agreement with a bank or trust company, as depositary, which will be named in the applicable prospectus supplement. Depositary shares will be evidenced by depositary receipts issued pursuant to the related deposit agreement. Immediately following our issuance of the security related to the depositary shares, NiSource will deposit the shares of our preferred stock with the relevant depositary and will cause the depositary to issue, on our behalf, the related depositary receipts. Subject to the terms of the deposit agreement, each owner of a depositary receipt will be entitled, in proportion to the fractional interest in the share of preferred stock represented by the related depositary share, to all the rights, preferences and privileges of, and will be subject to all of the limitations and restrictions on, the preferred stock represented by the depositary receipt (including, if applicable, dividend, voting, conversion, exchange, redemption, sinking fund, subscription and liquidation rights). The applicable prospectus supplement will describe the terms of the depositary shares offered thereby.

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DESCRIPTION OF THE DEBT SECURITIES

NiSource may issue debt securities, which will be designated as either senior debt securities, subordinated debt securities or junior subordinated debt securities, in one or more series from time to time. Unless the context requires otherwise, references to "debt securities" refer collectively to the senior debt securities, the subordinated debt securities and the junior subordinated debt securities. The senior debt securities will be issued under an indenture, dated as of November 14, 2000, as amended and supplemented, between NiSource (as successor to NiSource Finance Corp.) and The Bank of New York Mellon (as successor in interest to JPMorgan Chase Bank, N.A., formerly known as The Chase Manhattan Bank), as trustee. We refer to this indenture as the "Senior Indenture." The subordinated debt securities and junior subordinated debt securities will be issued under a separate indenture dated as of May 16, 2024, as amended and supplemented, between NiSource and The Bank of New York Mellon, as trustee. We refer to this indenture as the "Subordinated Indenture" and, together with the Senior Indenture, as the "Indentures." The Bank of New York Mellon, as trustee under the Indentures, will act as indenture trustee for the purposes of the Trust Indenture Act. We have filed the Indentures as exhibits to the registration statement of which this prospectus is a part.

This section briefly summarizes some of the terms of the debt securities and the Indentures. This section does not contain a complete description of the debt securities or the Indentures. The description of the debt securities is qualified in its entirety by the provisions of the Indentures. References to section numbers in this description of the debt securities, unless otherwise indicated, are references to section numbers of each Indenture.

General

The Indentures do not limit the amount of debt securities that may be issued. Each Indenture provides for the issuance of debt securities from time to time in one or more series. The terms of each series of debt securities may be established in a supplemental indenture or pursuant to resolutions of NiSource's board of directors or a committee of the board and set forth in an officers' certificate.

The senior debt securities:

The subordinated debt securities:

The indenture trustee has two principal functions under each Indenture. First, the indenture trustee can enforce your rights against NiSource if it defaults. However, there are limitations on the extent to which the indenture trustee may act on your behalf, which are described below under the heading "Information Concerning the Indenture Trustee." Second, the indenture trustee performs administrative duties for NiSource, including the delivery of interest and other payments and notices.

NiSource is a holding company with no independent business operations or source of income of its own. It conducts substantially all of its operations through its subsidiaries and, as a result, NiSource depends on the earnings and cash flow of, and dividends or distributions from, its subsidiaries to provide the funds necessary to meet its debt and contractual obligations. Substantially all of NiSource's consolidated assets, earnings and cash flow is derived from the operation of its regulated utility subsidiaries, whose legal authority to pay dividends or make other distributions to NiSource is subject to regulatory restrictions.

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NiSource's holding company status also means that its right to participate in any distribution of the assets of any of its subsidiaries upon liquidation, reorganization or otherwise is subject to the prior claims of the creditors of each of the subsidiaries (except to the extent that the claims of NiSource itself as a creditor of a subsidiary may be recognized). Since this is true for NiSource, it is also true for the creditors of NiSource (including the holders of the debt securities).

If NiSource uses this prospectus to offer debt securities, an accompanying prospectus supplement will describe the following terms of the debt securities being offered, to the extent applicable:

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The Indentures do not give holders of debt securities protection in the event of a highly leveraged transaction or other transaction involving NiSource. The Indentures also do not limit the ability of NiSource to incur indebtedness or to declare or pay dividends on its capital stock.

Conversion Rights

The terms, if any, on which a series of debt securities may be exchanged for or converted into shares of common stock or preferred stock of NiSource will be set forth in the applicable prospectus supplement.

Denomination, Registration and Transfer

NiSource may issue the debt securities as registered securities in certificated form or as global securities as described under the heading "Book-Entry Issuance." Unless otherwise specified in the applicable prospectus supplement, NiSource will issue registered debt securities in minimum denominations of $1,000 or any integral multiple thereof, in the case of the Senior Indenture, and in minimum denominations of $2,000 or any integral multiple thereof, in the case of the Subordinated Indenture. (See Section 302.)

If NiSource issues the debt securities as registered securities, NiSource will keep at one of its offices or agencies a register in which it will provide for the registration and transfer of the debt securities. NiSource will appoint that office or agency the security registrar for the purpose of registering and transferring the debt securities.

The holder of any registered debt security may exchange the debt security for registered debt securities of the same series having the same stated maturity date and original issue date, in any authorized denominations, in like tenor and in the same aggregate principal amount. The holder may exchange those debt securities by surrendering them in a place of payment maintained for this purpose at the office or agency NiSource has appointed as securities registrar. Holders may present the debt securities for exchange or registration of transfer, duly endorsed or accompanied by a duly executed written instrument of transfer satisfactory to NiSource and the securities registrar. No service charge will apply to any exchange or registration of transfer, but NiSource may require payment of any taxes and other governmental charges as described in the applicable Indenture. (See Section 305.)

If debt securities of any series are redeemed, NiSource will not be required to issue, register transfer of or exchange any debt securities of that series during the 15 business day period immediately preceding the day the relevant notice of redemption is given. That notice will identify the serial numbers of the debt securities being redeemed. After notice is given, NiSource will not be required to issue, register the transfer of or exchange any debt securities that have been selected to be either partially or fully redeemed, except the unredeemed portion of any debt security being partially redeemed. (See Section 305.)

Payment and Paying Agents

Unless otherwise indicated in the applicable prospectus supplement, on each interest payment date, NiSource will pay interest on each debt security to the person in whose name that debt security is registered as of the close of business on the record date relating to that interest payment date. If NiSource defaults in the payment of interest on any debt security, it may pay that defaulted interest to the registered owner of that debt security:

(See Section 307.)

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Unless otherwise indicated in the applicable prospectus supplement, NiSource will pay the principal of and any premium or interest due on the debt securities at maturity when they are presented at the office of the indenture trustee, as paying agent. NiSource may change the place of payment of the debt securities, appoint one or more additional paying agents, and remove any paying agent.

Redemption

The applicable prospectus supplement will contain the specific terms on which NiSource may redeem a series of debt securities prior to its stated maturity. NiSource will send a notice of redemption to holders at least 30 days but not more than 60 days prior to the redemption date, in the case of a redemption of a series of debt securities issued pursuant to the Senior Indenture, or at least 10 days but not more than 60 days prior to the redemption date, in the case of a redemption of a series of debt securities issued pursuant to the Subordinated Indenture. The notice will state:

(See Section 1104.)

On or before any redemption date, NiSource will deposit an amount of money with the indenture trustee or with a paying agent sufficient to pay the redemption price and any accrued interest, if any, on the debt securities to be redeemed. (See Section 1105.)

If NiSource is redeeming less than all the debt securities, the indenture trustee will select the debt securities to be redeemed using a method it considers fair and appropriate, in the case of a redemption of a series of debt securities issued pursuant to the Senior Indenture, or by lot or by such other customary method prescribed by the depositary, if applicable, in the case of a redemption of a series of debt securities issued pursuant to the Subordinated Indenture. After the redemption date, holders of redeemed debt securities will have no rights with respect to the debt securities except the right to receive the redemption price and any unpaid interest to the redemption date. (See Sections 1103 and 1106.)

Consolidation, Merger, Conveyance, Transfer or Lease

NiSource shall not consolidate with or merge into any other person or convey, transfer or lease substantially all of its assets or properties to any person unless:

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(See Section 801.)

Limitation on Liens

As long as any debt securities remain outstanding, neither NiSource nor any subsidiary of NiSource, other than a utility, may issue, assume or guarantee any debt for money borrowed secured by any mortgage, security interest, pledge, lien or other encumbrance on any property owned by NiSource or that subsidiary, except intercompany indebtedness, without also securing the debt securities (together with any other indebtedness of or guaranteed by NiSource or such subsidiary ranking equally with such debt securities) equally and ratably with (or prior to) the new debt, unless the total amount of all of the secured debt would not exceed 10% of the consolidated net tangible assets of NiSource and its subsidiaries (other than utilities).

The lien limitations do not apply to NiSource's and any subsidiary's ability to do the following:

(See Section 1008.)

Events of Default

The Indentures provide, with respect to any outstanding series of debt securities, that any of the following events constitutes an "Event of Default":

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(See Section 501.)

If an Event of Default occurs with respect to debt securities of a particular series, the indenture trustee or the holders of 33% in principal amount of the outstanding debt securities of that series may declare the debt securities of that series due and payable immediately. (See Section 502.)

The holders of a majority in principal amount of the outstanding debt securities of a particular series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the indenture trustee under the applicable Indenture, or exercising any trust or power conferred on the indenture trustee with respect to the debt securities of that series. The indenture trustee may refuse to follow directions that are in conflict with any rule of law or the applicable Indenture, that expose the indenture trustee to personal liability or that are unduly prejudicial to other holders. The indenture trustee may take any other action it deems proper that is not inconsistent with those directions. (See Section 512.)

The holders of a majority in principal amount of the outstanding debt securities of any series may waive any past default under the applicable Indenture and its consequences, except a default:

(See Section 513.)

At any time after the holders of the debt securities of a series declare that the debt securities of that series are due and immediately payable, holders of a majority in principal amount of the outstanding debt securities of that series may rescind and cancel the declaration and its consequences: (1) before the indenture trustee has obtained a judgment or decree for money, (2) if all events of default (other than the non-payment of principal which has become due solely by reason of the declaration) have been waived or cured, and (3) if NiSource has paid or deposited with the indenture trustee an amount sufficient to pay:

(See Section 502.)

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NiSource is required to furnish to the indenture trustee an annual certificate as to compliance with all terms, provisions and conditions under the applicable Indenture, and specifying any defaults and the nature and status thereof, within 120 days of the end of each fiscal year.

(See Section 1009.)

Modification of Indentures

NiSource and the indenture trustee may modify or amend one or both of the Indentures, without the consent of the holders of any debt securities, for any of the following purposes:

(See Section 901.)

Each Indenture provides that NiSource and the indenture trustee may amend the Indenture or the debt securities with the consent of the holders of a majority in principal amount of the then outstanding debt securities of each series affected by the amendment voting as one class. However, without the consent of each holder of any outstanding debt securities affected, an amendment or modification may not, among other things:

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(See Section 902.)

Satisfaction and Discharge

Under the Indentures, NiSource can terminate its obligations with respect to debt securities of all series not previously delivered to the indenture trustee for cancellation when those debt securities:

NiSource may terminate its obligations with respect to the debt securities of that series by depositing with the indenture trustee, as trust funds dedicated solely for that purpose, an amount sufficient to pay and discharge the entire indebtedness on the debt securities of that series. In that case, the applicable Indenture will cease to be of further effect and NiSource's obligations will be satisfied and discharged with respect to that series (except as to NiSource's obligations to pay all other amounts due under the applicable Indenture and to provide certain officers' certificates and opinions of counsel to the indenture trustee). At the expense of NiSource, the indenture trustee will execute proper instruments acknowledging the satisfaction and discharge. (See Section 401.)

Governing Law

Each of the Indentures is, and the related senior debt securities and subordinated debt securities will be, governed by the internal laws of the State of New York.

Information Concerning the Indenture Trustee

Prior to default, the indenture trustee will perform only those duties specifically set forth in the Indentures. After default, the indenture trustee will exercise the same degree of care as a prudent individual would exercise in the conduct of his or her own affairs. The indenture trustee is not required to expend or risk its own funds or otherwise incur personal financial liability in the performance of its duties if it reasonably believes that it may not receive repayment or adequate indemnity. (See Section 601.)

Because The Bank of New York Mellon is the trustee under the Senior Indenture and the Subordinated Indenture, it may be required to resign as trustee under one of those Indentures if there is an event of default under an Indenture.

We may appoint an alternative trustee for any series of debt securities. The appointment of an alternative trustee would be described in the applicable prospectus supplement.

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DESCRIPTION OF WARRANTS

NiSource may issue warrants to purchase equity or debt securities. NiSource may issue warrants independently or together with any offered securities. The warrants may be attached to or separate from those offered securities. NiSource will issue the warrants under warrant agreements to be entered into between NiSource and a bank or trust company, as warrant agent, all as described in the applicable prospectus supplement. The warrant agent will act solely as agent in connection with the warrants and will not assume any obligation or relationship of agency or trust for or with any holders or beneficial owners of warrants.

The prospectus supplement relating to any warrants that we may offer will contain the specific terms of the warrants. These terms may include the following:

Exercise of Warrants

Each warrant will entitle the holder of warrants to purchase for cash the amount of equity or debt securities at the exercise price stated or determinable in the prospectus supplement for the warrants. Warrants may be exercised at any time up to the close of business on the expiration date shown in the applicable prospectus supplement, unless otherwise specified in such prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void. Warrants may be exercised as described in the applicable prospectus supplement. When the warrant holder makes the payment and properly completes and signs the warrant certificate at the corporate trust office of the warrant agent or any other office indicated in the prospectus supplement, NiSource will, as soon as possible, forward the equity or debt securities that the warrant holder has purchased. If the warrant holder exercises the warrant for less than all of the warrants represented by the warrant certificate, NiSource will issue a new warrant certificate for the remaining warrants.

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DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

NiSource may issue stock purchase contracts, including contracts obligating holders to purchase from NiSource, and for NiSource to sell to the holders, a specified number of shares of common stock or preferred stock at a future date or dates ("Stock Purchase Contracts"). The price per share of common stock and the number of shares of common stock may be fixed at the time the stock purchase contracts are issued or may be determined by reference to a specific formula stated in the stock purchase contracts.

The stock purchase contracts may be issued separately or as part of units that we call "stock purchase units." Stock purchase units consist of a stock purchase contract and either NiSource's debt securities or U.S. treasury securities, securing the holders' obligations to purchase the shares of our common stock or preferred stock under the stock purchase contracts.

The stock purchase contracts may require us to make periodic payments to the holders of the stock purchase units or vice versa, and these payments may be unsecured or prefunded on some basis. The stock purchase contracts may require holders to secure their obligations in a specified manner.

The applicable prospectus supplement will describe the terms of the stock purchase contracts or stock purchase units. The description in the prospectus supplement will only be a summary, and you should read the stock purchase contracts, and, if applicable, collateral or depositary arrangements, relating to the stock purchase contracts or stock purchase units. Material U.S. federal income tax considerations applicable to the stock purchase units and the stock purchase contracts will also be discussed in the applicable prospectus supplement.

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BOOK-ENTRY ISSUANCE

Unless otherwise specified in the applicable prospectus supplement, NiSource will issue any debt securities offered under this prospectus as "global securities." In addition, NiSource may issue other securities offered under this prospectus as global securities. We will describe the specific terms for issuing any security as a global security in the prospectus supplement relating to that security.

Unless otherwise specified in the applicable prospectus supplement, The Depository Trust Company ("DTC"), will act as the depositary for any global securities. NiSource will issue global securities as fully registered securities registered in the name of DTC's nominee, Cede & Co. NiSource will issue one or more fully registered global securities for each issue of securities, each in the aggregate principal, stated amount or number of shares of such issue, and will deposit the global securities with DTC.

DTC is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code, and a "clearing agency" registered under the provisions of Section 17A of the Exchange Act. DTC holds and provides asset servicing for U.S. and non-U.S. equity issues, corporate and municipal debt issues and money market instruments that DTC's participants deposit with DTC. DTC also facilitates the post-trade settlement among its direct participants of sales and other securities transactions in deposited securities, through electronic computerized book-entry transfers and pledges between its direct participants' accounts. This eliminates the need for physical movement of securities certificates. DTC's direct participants include both U.S. and non-U.S. securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations. Access to the DTC system is also available to others such as U.S. and non-U.S. securities brokers and dealers, banks, trust companies and clearing corporations that clear through or maintain a custodial relationship with a DTC participant, either directly or indirectly. The DTC rules applicable to its participants are on file with the SEC.

Purchases of securities under DTC's system must be made by or through a direct participant, which will receive a credit for such securities on DTC's records. The ownership interest of each actual purchaser of each security, the beneficial owner, is in turn to be recorded on the records of direct and indirect participants. Beneficial owners will not receive written confirmation from DTC of their purchases, but they should receive written confirmations providing details of the transactions, as well as periodic statements of their holdings, from the participants through which they entered into the transactions. Transfers of ownership interests in the securities are to be accomplished by entries made on the books of participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in securities, except in the event that use of the book-entry system for the securities is discontinued.

To facilitate subsequent transfers, all global securities that are deposited with, or on behalf of, DTC are registered in the name of DTC's nominee, Cede & Co., or such other name as may be requested by an authorized representative of DTC. The deposit of global securities with, or on behalf of, DTC and their registration in the name of Cede & Co. or such other DTC nominee do not effect any changes in beneficial ownership. DTC has no knowledge of the actual beneficial owners of the securities; DTC's records reflect only the identity of the direct participants to whose accounts such securities are credited, which may or may not be the beneficial owners. The participants will remain responsible for keeping account of their holdings on behalf of their customers.

Conveyance of notices and other communications by DTC to direct participants, by direct participants to indirect participants and by direct and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

Redemption notices will be sent to DTC. If less than all of the securities of like type, tenor and terms are being redeemed, DTC's practice is to determine by lot the amount of the interest of each direct participant in such issue to be redeemed.

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Neither DTC nor Cede & Co. (nor any other DTC nominee) will consent or vote with respect to the global securities unless authorized by a direct participant in accordance with DTC's procedures. Under its usual procedures, DTC mails an omnibus proxy to NiSource as soon as possible after the applicable record date. The omnibus proxy assigns Cede & Co.'s consenting or voting rights to those direct participants to whose accounts the securities are credited on the applicable record date (identified in a listing attached to the omnibus proxy).

Redemption proceeds, principal payments and any premium, interest or other payments on the global securities will be made to Cede & Co., or such other nominee as may be requested by an authorized representative of DTC. DTC's practice is to credit direct participants' accounts upon DTC's receipt of funds and corresponding detail information on the payable date in accordance with their respective holdings shown on DTC's records. Payments by participants to beneficial owners will be governed by standing instructions and customary practices, as is the case with securities held for the accounts of customers registered in "street name," and will be the responsibility of the participant and not of DTC, NiSource or the indenture trustee, subject to any statutory or regulatory requirements. Payment of redemption proceeds, principal and any premium, interest or other payments to Cede & Co. (or such other nominee as may be requested by an authorized representative of DTC) is the responsibility of NiSource and the applicable paying agent, disbursement of payments to direct participants will be the responsibility of DTC, and disbursement of payments to the beneficial owners will be the responsibility of direct and indirect participants.

Except as provided in the applicable prospectus supplement, a beneficial owner will not be entitled to receive physical delivery of a security. Accordingly, each beneficial owner must rely on the procedures of DTC to exercise any rights with respect to such beneficial owner's interest in a global security. The laws of some jurisdictions require that certain purchasers of securities take physical delivery of securities in definitive form. Such laws may impair the ability to transfer beneficial interests in the global securities.

DTC may discontinue providing its services as securities depositary with respect to the global securities at any time by giving reasonable notice to NiSource or, with respect to a debt security, the indenture trustee. Under such circumstances, in the event that a successor securities depositary is not obtained, certificates for the securities are required to be printed and delivered to the holders of record.

NiSource may decide to discontinue use of the system of book-entry transfers through DTC (or a successor securities depositary). NiSource understands, however, that under current industry practices, DTC would notify its participants of NiSource's decision, but will only withdraw beneficial interests from the global securities at the request of each participant. In that event, certificates for the securities will be printed and delivered to the applicable participants.

The information in this section concerning DTC and DTC's book-entry system has been obtained from sources that we believe to be reliable, but we take no responsibility for the accuracy of this information. We have no responsibility for the performance by DTC or its participants of their respective obligations as described herein or under the rules and procedures governing their respective operations.

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PLAN OF DISTRIBUTION

We may sell the securities to or through underwriters, through dealers or agents, directly to you or through a combination of these methods. The prospectus supplement with respect to any offering of securities will describe the specific plan of distribution for the securities being offered, including:

Through Underwriters. If we use underwriters in the sale of the securities, the underwriters will acquire the offered securities for their own account. We will execute an underwriting agreement with an underwriter or underwriters once an agreement for sale of the securities is reached. The underwriters may resell the offered securities in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The underwriters may sell the offered securities directly or through underwriting syndicates represented by managing underwriters. Unless otherwise stated in the prospectus supplement relating to offered securities, the obligations of the underwriters to purchase those offered securities will be subject to certain conditions, and the underwriters will be obligated to purchase all of those offered securities if they purchase any of them.

Through Dealers. If we use a dealer to sell the securities, we will sell the offered securities to the dealer as principal. The dealer may then resell those offered securities at varying prices determined at the time of resale. Any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

Through Agents. If we use agents in the sale of securities, we may designate one or more agents to sell offered securities.

Directly to Purchasers. We may sell the offered securities directly to one or more purchasers. In this case, no underwriters, dealers or agents would be involved. We will describe the terms of our direct sales in our prospectus supplement.

General Information. A prospectus supplement will state the name of any underwriter, dealer or agent and the amount of any compensation, underwriting discounts or concessions paid, allowed or reallowed to them. A prospectus supplement will also state the proceeds to us from the sale of offered securities, any initial public offering price and other terms of the offering of those offered securities.

Our agents, underwriters and dealers, or their affiliates, may be customers of, engage in transactions with or perform services for us in the ordinary course of business.

We may authorize agents, underwriters or dealers to solicit offers by certain institutions to purchase offered securities from us at the public offering price and on terms described in the related prospectus supplement pursuant to delayed delivery or forward contracts providing for payment and delivery on a specified date in the future. If we use delayed delivery contracts, we will disclose that we are using them in our prospectus

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supplement and will tell you when we will demand payment and delivery of the securities. The delayed delivery contracts will be subject only to the conditions we set forth in our prospectus supplement.

We may enter into agreements to indemnify agents, underwriters and dealers against certain civil liabilities, including liabilities under the Securities Act.

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LEGAL MATTERS

McGuireWoods LLP, Charlotte, North Carolina, will pass upon certain legal matters relating to the validity of the securities offered by this prospectus for us. Any underwriters or sales agents will be represented by their own legal counsel, which will be named in the applicable prospectus supplement.

EXPERTS

The consolidated financial statements, and the related financial statement schedule, incorporated by reference in this prospectus from the NiSource Inc. Annual Report on Form 10-K for the year ended December 31, 2024, and the effectiveness of NiSource Inc. and subsidiaries' internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such consolidated financial statements and financial statement schedule have been so incorporated in reliance upon the reports of such firm given their authority as experts in accounting and auditing.

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$    

NiSource Inc.

$    % Notes due 2031

$    % Notes due 2036

Prospectus Supplement

Joint Book-Running Managers

BNP PARIBAS

BofA Securities

MUFG

US Bancorp

Huntington Capital Markets

KeyBanc Capital Markets

PNC Capital Markets LLC

Scotiabank

May , 2026