Supplemental Prospectus (Form 424B5)

 Filed Pursuant to Rule 424(b)(5)​

 Registration No. 333-275300​

PROSPECTUS SUPPLEMENT
(To Prospectus dated November 3, 2023)

10,345,000 Shares of Common Stock

We are offering 10,345,000 shares of our common stock in this offering.

Our common stock is listed on the Nasdaq Capital Market ("Nasdaq") under the symbol "CLDX." The last reported sale price of our common stock on April 1, 2026 was $31.26 per share.

Investing in our common stock involves a high degree of risk. Before buying shares of our common stock, you should carefully consider the risk factors described in "Risk Factors" beginning on page S-8 of this prospectus supplement and in the risks discussed under similar headings in the documents incorporated by reference into this prospectus supplement, the accompanying prospectus and any free writing prospectus that we have authorized for use in connection with this offering.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

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(1)

See the section entitled "Underwriting" beginning on page S-12 for a description of the compensation payable to the underwriters.

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We have granted to the underwriters an option for a period of 30 days to purchase up to an additional 1,551,750 shares of our common stock from us at the public offering price set forth above, less the underwriting discounts and commissions.

The underwriters expect to deliver the shares of common stock against payment on or about April 6, 2026.

Joint Bookrunning Managers

Co-Lead Managers

The date of this prospectus supplement is April 1, 2026.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus. The second part, the accompanying prospectus, provides more general information, some of which may not apply to this offering. Generally, when we refer to this prospectus supplement, we are referring to both parts of this document combined. To the extent there is a conflict between the information contained in this prospectus supplement and the information contained in the accompanying prospectus or any document incorporated by reference that was filed with the U.S. Securities and Exchange Commission (the "SEC") before the date of this prospectus supplement, you should rely on the information in this prospectus supplement; provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date - for example, a document incorporated by reference in the accompanying prospectus - the statement in the document having the later date modifies or supersedes the earlier statement.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

Neither we nor the underwriters have authorized anyone to provide information different from that contained in this prospectus supplement and the accompanying prospectus, including any free writing prospectus that we have authorized for use in connection with this offering. When you make a decision about whether to invest in our common stock, you should not rely upon any information other than the information in this prospectus supplement and the accompanying prospectus, including any free writing prospectus that we have authorized for use in connection with this offering. Neither the delivery of this prospectus supplement nor the accompanying prospectus, including any free writing prospectus that we have authorized for use in connection with this offering, nor the sale of our common stock means that information contained in this prospectus supplement and the accompanying prospectus, including any free writing prospectus that we have authorized for use in connection with this offering, is correct after their respective dates. The information contained in this prospectus supplement or the accompanying prospectus, or incorporated by reference herein or therein is accurate only as of the respective dates thereof, regardless of the time of delivery of this prospectus supplement and the accompanying prospectus or of any sale of our common stock. It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus, including the information incorporated by reference herein or therein, and any free writing prospectus that we have authorized for use in connection with this offering in making your investment decision. You should also read and consider the information in the documents to which we have referred you in the sections entitled "Where You Can Find More Information" and "Incorporation of Certain Documents by Reference" in this prospectus supplement and in the accompanying prospectus.

We are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation of any offer in such jurisdiction.

Unless otherwise indicated in this prospectus supplement or the context otherwise requires, all references to "we," "us," "our," "the Company," and "Celldex" refer to Celldex Therapeutics, Inc. and its consolidated subsidiary.

FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein contain forward-looking statements made pursuant to the safe harbor provisions of the Private Securities Litigation Reform Act of 1995. Forward-looking statements represent our management's judgment regarding future events. In many cases, you can identify these forward-looking statements through our use of words such as "may," "will," "can," "anticipate," "assume," "should," "indicate," "would," "believe," "contemplate," "expect," "seek," "estimate," "continue," "plan," "point to," "project," "predict," "could," "intend," "target," "potential" and other similar words and expressions of the future. All statements other than statements of historical fact included in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein regarding our financial position, business strategy and plans or objectives for future operations are forward-looking statements. Without limiting the broader description of forward-looking statements above, we specifically note that statements regarding potential drug candidates, their potential therapeutic effect, the possibility of obtaining regulatory approval, our expected timing for completing clinical trials and clinical trial milestones for our drug candidates, our ability or the ability of our collaborators to manufacture and sell any products, market acceptance or our ability to earn a profit from sales or licenses of any drug candidate or to discover new drugs in the future are all forward-looking in nature.

There are a number of important factors that could cause the actual results to differ materially from those expressed in any forward-looking statement made by us. These factors include, but are not limited to:

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our dependence on product candidates that are still in development stages;

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our ability to successfully complete research and further development, including preclinical and clinical studies;

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our anticipated timing for preclinical development, regulatory submissions, commencement and completion of clinical trials and product approvals;

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our ability to commercialize our drug candidates and the growth of the markets for those drug candidates;

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our ability to develop and commercialize products before competitors that are superior to the alternatives developed by such competitors;

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our ability to negotiate strategic partnerships, where appropriate, for our drug candidates;

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our ability to manage multiple clinical trials for a variety of drug candidates at different stages of development;

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the cost, timing, scope and results of ongoing preclinical and clinical testing;

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our expectations of the attributes of our product and development candidates, including pharmaceutical properties, efficacy, safety and dosing regimens;

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the cost, timing and uncertainty of obtaining regulatory approvals for our drug candidates;

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the availability, cost, delivery and quality of clinical management services provided by our clinical research organization partners;

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the availability, cost, delivery and quality of clinical and commercial-grade materials produced by our own manufacturing facility or supplied by contract manufacturers, suppliers and partners;

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our ability to develop technological capabilities, including identification of novel and clinically important targets, exploiting our existing technology platforms to develop new drug candidates and expand our focus to broader markets for our existing targeted therapeutics;

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the cost of paying the regulatory approval milestone under the merger agreement by which we acquired Kolltan Pharmaceuticals, Inc. ("Kolltan") and our related settlement agreement with Kolltan;

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our ability to raise sufficient capital to fund our preclinical and clinical studies and to meet our long-term liquidity needs, on terms acceptable to us, or at all. If we are unable to raise the funds

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necessary to meet our long-term liquidity needs, we may have to delay or discontinue the development of one or more programs, discontinue or delay ongoing or anticipated clinical trials, discontinue or delay our commercial manufacturing efforts, discontinue or delay our efforts to expand into additional indications for our drug product candidates, license out programs earlier than expected, raise funds at significant discount or on other unfavorable terms, if at all, or sell all or part of our business;

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our ability to protect our intellectual property rights and our ability to avoid intellectual property litigation, which can be costly and divert management time and attention;

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our ability to develop and commercialize products without infringing the intellectual property rights of third parties; and

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the anticipated use of the net proceeds from this offering.

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You should also consider carefully the statements set forth in the section entitled "Risk Factors" in this prospectus supplement and in our Annual Report on Form 10-K for the year ended December 31, 2025, as updated by any other document that we subsequently file with the SEC and that is incorporated by reference into this prospectus supplement, which address various factors that could cause results or events to differ from those described in the forward-looking statements. All subsequent written and oral forward-looking statements attributable to us or to persons acting on our behalf are expressly qualified in their entirety by the applicable cautionary statements. We undertake no obligation to make any revisions to the forward-looking statements contained in this prospectus supplement or to update them to reflect events or circumstances occurring after the date hereof except as required by applicable law.

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RISK FACTORS

Investment in our common stock involves risks. Before deciding whether to invest in our common stock, you should consider carefully the risk factors discussed below and those contained in the section entitled "Risk Factors" contained in our Annual Report on Form 10-K for the year ended December 31, 2025, which is incorporated herein by reference in its entirety, together with other information in the accompanying prospectus, the information and documents incorporated by reference herein and therein, and in any free writing prospectus that we have authorized for use in connection with this offering. If any of the risks or uncertainties described in our SEC filings actually occurs, our business, financial condition, results of operations or cash flow could be materially and adversely affected. This could cause the trading price of our common stock to decline, resulting in a loss of all or part of your investment. The risks and uncertainties we have described are not the only ones facing our company. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business operations.

Risks Associated with this Offering

We have broad discretion in the use of the net proceeds of this offering and may not use them effectively.

We intend to use the net proceeds of this offering, together with our existing cash, cash equivalents, and marketable securities, (i) to fund ongoing commercial readiness activities and the commercial launch of barzolvolimab, if approved, for the treatment of CSU in the United States, (ii) to continue the clinical and preclinical development of our product candidates, including current and future development of barzolvolimab, (iii) to grow our bispecific antibody platform and clinical candidates, (iv) to fund ongoing efforts to develop additional clinical pipeline product candidates and (v) for general corporate purposes. However, our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations or enhance the value of our common stock. The failure by management to apply these funds effectively could result in financial losses that could have a material adverse effect on our business, cause the price of our common stock to decline and delay the development of our product candidates.

If you purchase shares of common stock sold in this offering, you will experience immediate and substantial dilution in the book value per share of the common stock you purchase.

The price of our common stock to be sold in this offering is substantially higher than the net tangible book value per share of our common stock. If you purchase common stock in this offering, you will experience immediate dilution of $18.83 per share, representing the difference between as adjusted net tangible book value per share after giving effect to the sale by us of shares in this offering and at the public offering price of $29.00 per share. See the section entitled "Dilution" below for a more detailed discussion of the dilution you will incur if you purchase common stock in this offering. In addition, we have a significant number of options outstanding. If the holders of these securities exercise them or become vested in them, as applicable, you may incur further dilution.

You may experience future dilution as a result of future equity offerings, or if we elect to pay the future milestone payment, if any, due to former stockholders of Kolltan Pharmaceuticals, Inc., or Kolltan, in shares of our common stock.

To raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share in this offering. We may sell shares or other securities in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering.

In addition, in connection with the agreement and plan of merger pursuant to which we acquired Kolltan in 2016 (the "Merger Agreement"), as modified by the related definitive settlement agreement (the "Settlement Agreement") we entered into on July 15, 2022 related to litigation arising from the Kolltan merger, in the event that regulatory approval by the United States Food and Drug Administration or European

Medicines Agency of certain drug candidates are achieved, we will be required to pay to the former stockholders of Kolltan a milestone payment of $52.5 million, which milestone payment may be made, at our sole election, in cash, in shares of our common stock or a combination of both, subject to provisions of the Merger Agreement.

We may require additional capital to fund the milestone payment in cash, depending on the facts and circumstances at the time such payment becomes due. The number of shares of our common stock issuable in connection with a milestone payment, if any, will be determined based on the average closing price per share of our common stock for the five trading day period ending three calendar days prior to the achievement of such milestone. If we elect to issue shares of our common stock to make the milestone payment, you will experience further dilution.

Our share price has been and could remain volatile.

The market price of our common stock has historically experienced and may continue to experience significant volatility. From January 2024 through December 2025, the market price of our common stock has fluctuated from a high of $53.18 per share in the first quarter of 2024, to a low of $14.40 per share in the second quarter of 2025. Our progress in developing and commercializing our products, the impact of government regulations on our products and industry, the potential sale of a large volume of our common stock by stockholders, our quarterly operating results, changes in general conditions in the economy or the financial markets and other developments affecting us or our competitors could cause the market price of our common stock to fluctuate substantially with significant market losses. If our stockholders sell a substantial number of shares of common stock, especially if those sales are made during a short period of time, those sales could adversely affect the market price of our common stock and could impair our ability to raise capital. In addition, in recent years, the stock market has experienced significant price and volume fluctuations. This volatility has affected the market prices of securities issued by many companies for reasons unrelated to their operating performance and may adversely affect the price of our common stock. In addition, we could be subject to a securities class action litigation as a result of volatility in the price of our stock, which could result in substantial costs and diversion of management's attention and resources and could harm our stock price, business, prospects, results of operations and financial condition.

Sales of a significant number of shares of our common stock in the public markets, or the perception that such sales could occur, could depress the market price of our common stock.

Sales of a substantial number of shares of our common stock in the public markets, or the perception that such sales could occur, could depress the market price of our common stock and impair our ability to raise capital through the sale of additional equity securities. We may issue and sell additional shares of our common stock in the public markets including, without limitation, through our "at-the-market" offering program, underwritten public offerings, privately negotiated transactions, block trades, or any combination of the above. We cannot predict the effect that future sales of our common stock would have on the market price of our common stock.

Because we do not intend to declare cash dividends on our shares of common stock in the foreseeable future, stockholders must rely on appreciation of the value of our common stock for any return on their investment.

We have never declared or paid cash dividends on our common stock. We currently anticipate that we will retain future earnings for the development, operation and expansion of our business and do not anticipate declaring or paying any cash dividends in the foreseeable future. In addition, the terms of any existing or future debt agreements may preclude us from paying dividends. As a result, we expect that only appreciation of the price of our common stock, if any, will provide a return to investors in this offering for the foreseeable future.

USE OF PROCEEDS

The net proceeds to us from the sale of shares of our common stock will be approximately $281.8 million, or approximately $324.1 million if the option to purchase additional shares is exercised in full by the underwriters, in each case, after deducting underwriting discounts and commissions and estimated offering expenses payable by us.

We intend to use the net proceeds of this offering, together with our existing cash, cash equivalents, and marketable securities, (i) to fund ongoing commercial readiness activities and the commercial launch of barzolvolimab, if approved, for the treatment of CSU in the United States, (ii) to continue the clinical and preclinical development of our product candidates, including current and future development of barzolvolimab, (iii) to grow our bispecific antibody platform and clinical candidates, (iv) to fund ongoing efforts to develop additional clinical pipeline product candidates and (v) for general corporate purposes.

This expected use of the net proceeds from this offering represents our intentions based upon our current plans and business conditions, which could change in the future as our plans and business conditions evolve. The timing and amounts of our actual expenditures will depend on several factors, including the progress of our research and development programs, the results of other preclinical and clinical studies and the timing and costs of regulatory approvals. As a result, our management will have broad discretion over the use of proceeds from this offering. We may find it necessary or advisable to use the net proceeds from this offering for other purposes, and we will have broad discretion in the application of net proceeds. Pending the uses described above, we will invest the net proceeds in short-term and long-term, investment grade, interest-bearing securities.

DILUTION

If you invest in our common stock, your ownership interest will be diluted to the extent of the difference between the public offering price per share of common stock in this offering and the net tangible book value per share of our common stock immediately after this offering. Our net tangible book value as of December 31, 2025 was approximately $500.0 million, or $7.51 per share. Net tangible book value per share is equal to our total tangible assets, less our total liabilities, divided by the total number of shares outstanding as of December 31, 2025.

After giving effect to the issuance and sale of 10,345,000 shares of our common stock in this offering at an offering price of $29.00 per share, and after deducting underwriting discounts and commissions and estimated offering expenses payable by us, our net tangible book value as of December 31, 2025 would have been approximately $781.8 million, or approximately $10.17 per share of common stock. This represents an immediate increase in net tangible book value of $2.66 per share to our existing stockholders and an immediate dilution in net tangible book value of $18.83 per share to new investors in this offering.

The following table illustrates this calculation on a per share basis:

If the underwriters exercise in full their option to purchase additional shares of common stock in this offering at the public offering price of $29.00 per share, our net tangible book value after the offering would be approximately $824.1 million, or approximately $10.50 per share, the increase in the net tangible book value to existing stockholders would be $2.99 per share, and the dilution to new investors purchasing securities in this offering would be $18.50 per share.

The number of shares of our common stock to be outstanding immediately after this offering is based on 66,549,442 shares of our common stock outstanding as of December 31, 2025, and excludes the following, in each case as of such date:

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9,134,278 shares of common stock issuable upon the exercise of outstanding stock options as of December 31, 2025 with a weighted-average exercise price of $26.49 per share;

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2,753,587 shares of common stock reserved for future issuance under our equity compensation plans as of December 31, 2025; and

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any shares of our common stock we may elect to issue pursuant to the future milestone under the Merger Agreement and the related Settlement Agreement.

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To the extent that any options are exercised, new options are issued under our equity incentive plan or we otherwise issue additional shares of common stock in the future at a price less than the public offering price, there will be further dilution to new investors.

UNDERWRITING

Leerink Partners LLC and TD Securities (USA) LLC are acting as representatives of each of the underwriters named below and as joint bookrunning managers for this offering. Subject to the terms and conditions set forth in the underwriting agreement among us and the underwriters, we have agreed to sell to the underwriters, and each of the underwriters has agreed, severally and not jointly, to purchase from us, the number of shares of common stock set forth opposite its name below.

Subject to the terms and conditions set forth in the underwriting agreement, the underwriters have agreed, severally and not jointly, to purchase all of the shares sold under the underwriting agreement if any of the shares are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the non-defaulting underwriters may be increased or the underwriting agreement may be terminated.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act, or to contribute to payments the underwriters may be required to make in respect of those liabilities.

The underwriters are offering the shares, subject to prior sale, when, as and if issued to and accepted by them, subject to approval of legal matters by their counsel, including the validity of the shares, and subject to other conditions contained in the underwriting agreement, such as the receipt by the underwriters of officers' certificates and legal opinions. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Discounts and Commissions

The representatives have advised us that the underwriters propose initially to offer the shares to the public at the initial public offering price set forth on the cover page of this prospectus supplement and to dealers at that price less a concession not in excess of $1.044 per share. After the initial offering of the shares, the public offering price, concession or any other term of this offering may be changed by the representatives.

The following table shows the initial public offering price, underwriting discounts and commissions and proceeds, before expenses, to us. The information assumes either no exercise or full exercise by the underwriters of their option to purchase additional shares of our common stock.

We estimate expenses payable by us in connection with this offering, other than the underwriting discounts and commissions referred to above, will be approximately $220,000. We also have agreed to reimburse the underwriters for up to $15,000 for their FINRA counsel fee. In accordance with FINRA Rule 5110, this reimbursed fee is deemed underwriting compensation for this offering.

Option to Purchase Additional Shares

We have granted an option to the underwriters, exercisable for 30 days after the date of this prospectus supplement, to purchase up to 1,551,750 additional shares at the initial public offering price, less underwriting discounts and commissions. If the underwriters exercise this option, each underwriter will be obligated, subject to the conditions contained in the underwriting agreement, to purchase a number of additional shares proportionate to that underwriter's initial amount reflected in the above table.

No Sales of Similar Securities

We and our executive officers and directors have agreed, subject to specified exceptions, not to directly or indirectly, for a period of 60 days from the date of this prospectus supplement, without obtaining the prior written consent of Leerink Partners LLC and TD Securities (USA) LLC:

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sell, offer to sell, contract to sell or lend, effect any short sale or establish or increase a "put equivalent position" ​(as defined in Rule 16a-1(h) under the Securities Exchange Act of 1934, as amended (the "Exchange Act")), or liquidate or decrease any "call equivalent position" ​(as defined in Rule 16a-1(b) under the Exchange Act), pledge, hypothecate or grant any security interest in, or in any other way transfer or dispose of, any common stock or any securities convertible into or exchangeable or exercisable for common stock currently or thereafter owned either of record or beneficially;

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enter into any swap, hedge or any other agreement or any transaction that transfers, in whole or in part, the economic consequence of ownership of any common stock or any securities convertible into or exchangeable or exercisable for common stock, whether any such swap or transaction is to be settled by delivery of common stock or other securities, in cash or otherwise; or

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publicly announce an intention to do any of the foregoing.

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In addition, we and each such person have agreed that, without the prior written consent of Leerink Partners LLC and TD Securities (USA) LLC, we or such other person will not, during such 60-day period, make any demand for, or exercise any right with respect to, the registration of any shares of common stock or any security convertible into or exercisable or exchangeable for common stock.

Leerink Partners LLC and TD Securities (USA) LLC may, in their sole discretion and at any time or from time to time before the termination of the 60-day period release all or any portion of the securities subject to lock-up agreements. There are no existing agreements between the underwriters and any of our shareholders who will execute a lock-up agreement, providing consent to the sale of shares prior to the expiration of the lock-up period.

Nasdaq Global Market Listing

Our common stock is listed on the Nasdaq Capital Market under the symbol "CLDX."

Price Stabilization, Short Positions and Penalty Bids

Until the distribution of the shares is completed, SEC rules may limit underwriters and selling group members from bidding for and purchasing our common stock. However, the representatives may engage in transactions that stabilize the price of the common stock, such as bids or purchases to peg, fix or maintain that price.

In connection with this offering, the underwriters may purchase and sell our common stock in the open market. These transactions may include short sales, purchases on the open market to cover positions created by short sales and stabilizing transactions. Short sales involve the sale by the underwriters of a greater number of shares than they are required to purchase in this offering. "Covered" short sales are sales made in an amount not greater than the underwriters' option to purchase additional shares described above. The underwriters may close out any covered short position by either exercising their option to purchase additional shares or purchasing shares in the open market. In determining the source of shares to close out the covered short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase shares through the option to purchase additional shares granted to them under the underwriting agreement described above. "Naked"

short sales are sales in excess of such option. The underwriters must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of our common stock in the open market after pricing that could adversely affect investors who purchase in this offering. Stabilizing transactions consist of various bids for or purchases of shares of common stock made by the underwriters in the open market prior to the closing of this offering.

The underwriters may also impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the representatives have repurchased shares sold by or for the account of such underwriter in stabilizing or short covering transactions.

Similar to other purchase transactions, the underwriters' purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. The underwriters may conduct these transactions on the Nasdaq Capital Market, in the over-the-counter market or otherwise.

Neither we nor any of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. In addition, neither we nor any of the underwriters make any representation that the representatives will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Electronic Distribution

In connection with this offering, certain of the underwriters or securities dealers may distribute prospectuses by electronic means, such as e-mail.

Other Relationships

The underwriters and certain of their affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. Some of the underwriters and certain of their affiliates may in the future engage in investment banking and other commercial dealings in the ordinary course of business with us and our affiliates, for which they may in the future receive customary fees, commissions and expenses.

In addition, in the ordinary course of their business activities, the underwriters and their 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 account and for the accounts of their customers. Such investments and securities activities may involve securities and/or instruments of ours or our affiliates. The underwriters and their affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or financial instruments and may hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Selling Restrictions

Notice to Prospective Investors in the European Economic Area

In relation to each Member State of the European Economic Area (each, a "Relevant State"), no shares have been offered or will be offered pursuant to the offering to the public in that Relevant State prior to the publication of a prospectus in relation to the shares which has been approved by the competent authority in that Relevant State or, where appropriate, approved in another Relevant State and notified to the competent authority in that Relevant State, all in accordance with the Prospectus Regulation, except that shares may be offered to the public in that Relevant State at any time:

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to any legal entity which is a qualified investor as defined under Article 2 of the Prospectus Regulation;

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to fewer than 150 natural or legal persons (other than qualified investors as defined under Article 2 of the Prospectus Regulation), subject to obtaining the prior consent of the representatives for any such offer; or

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in any other circumstances falling within Article 1(4) of the Prospectus Regulation, provided that no such offer of shares shall require us or the representatives to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the Prospectus Regulation.

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For the purposes of this provision, the expression an "offer to the public" in relation to any shares in any Relevant State means the communication in any form and by any means of sufficient information on the terms of the offer and any shares to be offered so as to enable an investor to decide to purchase or subscribe for any shares, and the expression "Prospectus Regulation" means Regulation (EU) 2017/1129, as amended.

Notice to Prospective Investors in the United Kingdom

No shares have been offered or will be offered pursuant to the offering to the public in the United Kingdom prior to the publication of a prospectus in relation to the shares which has been approved by the Financial Conduct Authority, except that the shares may be offered to the public in the United Kingdom at any time:

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to any legal entity which is a qualified investor as defined under Article 2 of the UK Prospectus Regulation;

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to fewer than 150 natural or legal persons (other than qualified investors as defined under Article 2 of the UK Prospectus Regulation), subject to obtaining the prior consent of the representatives for any such offer; or

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in any other circumstances falling within Section 86 of the Financial Services and Markets Act 2000 (the "FMSA"),

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provided that no such offer of the shares shall require us or the representatives to publish a prospectus pursuant to Section 85 of the FSMA or supplement a prospectus pursuant to Article 23 of the UK Prospectus Regulation. For the purposes of this provision, the expression an "offer to the public" in relation to the shares in the United Kingdom means the communication in any form and by any means of sufficient information on the terms of the offer and any shares to be offered so as to enable an investor to decide to purchase or subscribe for any shares and the expression "UK Prospectus Regulation" means Regulation (EU) 2017/1129 as it forms part of domestic law by virtue of the European Union (Withdrawal) Act 2018.

Notice to Prospective Investors in Canada

The shares may be sold 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 shares 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 (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.

Notice to Prospective Investors Switzerland

The securities may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange ("SIX"), or on any other stock exchange or regulated trading facility in Switzerland. This prospectus supplement has been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither this prospectus supplement nor any other offering or marketing material relating to the securities or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this prospectus supplement nor any other offering or marketing material relating to the offering, us or the securities have been or will be filed with or approved by any Swiss regulatory authority. In particular, this prospectus supplement will not be filed with, and the offer of securities will not be supervised by, the Swiss Financial Market Supervisory Authority FINMA and the offer of securities has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes ("CISA"). The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of securities.

LEGAL MATTERS

The validity of the shares of common stock offered hereby will be passed upon by Lowenstein Sandler LLP, New York, New York. Wilmer Cutler Pickering Hale and Dorr LLP, New York, New York is counsel to the underwriters in connection with this offering.

EXPERTS

The financial statements and management's assessment of the effectiveness of internal control over financial reporting (which is included in Management's Annual Report on Internal Control over Financial Reporting) incorporated in this prospectus supplement by reference to the Annual Report on Form 10-K for the year ended December 31, 2025 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC a registration statement on Form S-3, including exhibits, under the Securities Act of which this prospectus supplement and the accompanying prospectus forms a part. This prospectus supplement and the accompanying prospectus do not contain all of the information set forth in the registration statement. This prospectus supplement and the accompanying prospectus are part of the registration statement but do not contain descriptions of certain agreements or documents that are exhibits to the registration statement. The statements as to the contents of such exhibits, however, are brief descriptions and are not necessarily complete, and each statement is qualified in all respects by reference to such agreement or document. For further information about us, please refer to the registration statement and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC's website at http://www.sec.gov. The SEC's website contains reports, proxy statements and other information regarding issuers, such as Celldex Therapeutics, Inc., that file electronically with the SEC. We make available free of charge through our web site our Annual Report on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, Proxy Statements on Schedule 14A and all amendments to those reports as soon as reasonably practicable after such material is electronically filed with or furnished to the SEC. Our website address is http://www.celldex.com. Please note that our website address is provided as an inactive textual reference only. Information contained on or accessible through our website is not part of this prospectus supplement or the accompanying prospectus, and is therefore not incorporated by reference unless such information is otherwise specifically referenced elsewhere in this prospectus supplement or the accompanying prospectus.

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to "incorporate by reference" into this prospectus supplement the information we have filed with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information that we file subsequently with the SEC will automatically update this prospectus supplement. We incorporate by reference into this prospectus supplement the information contained in the documents listed below, which is considered to be a part of this prospectus supplement:

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our Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 25, 2026;

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the information in our Definitive Proxy Statement on Schedule 14A, filed with the SEC on April 21, 2025, but only to the extent incorporated by reference into our Annual Report on Form 10-K for the year ended December 31, 2024; and

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the description of our common stock contained in our Registration Statement on Form 8-A, filed on November 8, 2004, as amended by Form 8-A/A filed on October 22, 2007 and March 7, 2008.

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We also incorporate by reference all documents we file under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act until the offering of the securities offered hereby is terminated or completed. The most recent information that we file with the SEC automatically updates and supersedes older information. The information contained in any such filing will be deemed to be a part of this prospectus supplement, commencing on the date on which the document is filed.

Nothing in this prospectus supplement shall be deemed to incorporate information furnished but not filed with the SEC pursuant to Item 2.02 or 7.01 of Form 8-K.

We will furnish without charge to each person, including any beneficial owner, to whom this prospectus supplement is delivered, upon written or oral request, a copy of any documents incorporated by reference other than exhibits to those documents. Requests should be addressed to:

Corporate Secretary
Celldex Therapeutics, Inc.
Perryville III Building, 53 Frontage Road, Suite 220,
Hampton, New Jersey 08827
(908) 200-7500

Any statement contained in this prospectus supplement or in a document incorporated or deemed to be incorporated by reference into this prospectus supplement will be deemed to be modified or superseded for purposes of this prospectus supplement to the extent that a statement contained in this prospectus supplement or any other subsequently filed document that is deemed to be incorporated by reference into this prospectus supplement modifies or supersedes the statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus supplement.

You should rely only on information contained in, or incorporated by reference into, this prospectus supplement and the accompanying prospectus. We have not authorized anyone to provide you with information different from that contained in this prospectus supplement and the accompanying prospectus or incorporated by reference in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters are making offers to sell the securities in any jurisdiction in which such an offer or solicitation is not authorized or in which the person making such offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make such offer or solicitation.

PROSPECTUS

Common Stock
Preferred Stock
Warrants
Depositary Shares
Units

Celldex Therapeutics, Inc. may offer, issue and sell from time to time, together or separately, in one or more offerings, any combination of:

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our common stock,

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our preferred stock, which we may issue in one or more series,

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warrants,

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depositary shares, and

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units.

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This prospectus provides a general description of the securities we may offer. Each time we or any selling securityholders sell securities, we will provide specific terms of the securities offered in a supplement to this prospectus. The prospectus supplement may also add to, update or change information contained in this prospectus. You should read this prospectus and the accompanying prospectus supplement, as well as the documents incorporated or deemed incorporated by reference in this prospectus, carefully before you make your investment decision. Our common stock is traded on the Nasdaq Capital Market under the symbol "CLDX." On November 2, 2023, the last reported sale price of our common stock on the Nasdaq Capital Market was $24.01 per share. You are urged to obtain current market quotations of the common stock. Each prospectus supplement will indicate if the securities offered thereby will be listed on any securities exchange.

This prospectus may not be used to sell securities unless accompanied by a prospectus supplement.

We or any selling securityholders may offer to sell these securities on a continuous or delayed basis, through agents, dealers or underwriters, or directly to purchasers. The prospectus supplement for each offering of securities will describe in detail the plan of distribution for that offering. If our agents or any dealers or underwriters are involved in the sale of the securities, the applicable prospectus supplement will set forth the names of the agents, dealers or underwriters and any applicable commissions or discounts. Our net proceeds from the sale of securities will also be set forth in the applicable prospectus supplement. For general information about the distribution of securities offered, please see "Plan of Distribution" in this prospectus.

Investing in our securities involves risks. Before making an investment decisions, you should carefully review the information contained in this prospectus under the heading "Risk Factors" beginning on page 4 of this prospectus.

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES COMMISSION OR REGULATORY BODY 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 November 3, 2023.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

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

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 registration statement and the accompanying exhibits for further information. The registration statement, including the exhibits and the documents incorporated or deemed incorporated herein by reference, can be read and are available to the public over the Internet at the SEC's website at http://www.sec.gov as described under the heading "Where You Can Find More Information."

This prospectus provides you with a general description of the securities we may offer. Each time we sell securities pursuant to this prospectus, we will provide a prospectus supplement (which term includes, as applicable, the at the market sale agreement prospectus filed with the registration statement of which this prospectus forms a part) containing specific information about the terms of a particular offering by us. That prospectus supplement may include a discussion of any risk factors or other special considerations that apply to those securities. The prospectus supplement may add, update or change information in this prospectus. If the information in the prospectus is inconsistent with a prospectus supplement, you should rely on the information in that prospectus supplement. You should read both this prospectus and, if applicable, any prospectus supplement. See "Where You Can Find More Information" for more information.

You should rely only on the information incorporated by reference or provided in this prospectus or any prospectus supplement. We have not authorized any dealer, salesman or other person to give any information or to make any representation other than those contained or incorporated by reference in this prospectus or any prospectus supplement. You must not rely upon any information or representation not contained or incorporated by reference in this prospectus or any prospectus supplement. This prospectus and any prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor do this prospectus and any prospectus supplement constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction. You should not assume that the information contained in this prospectus or any prospectus supplement is accurate on any date subsequent to the date set forth on the front of such document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus and any prospectus supplement is delivered or securities are sold on a later date.

Unless this prospectus indicates otherwise or the context otherwise requires, the terms "we," "our," "us," "Celldex" or the "Company" as used in this prospectus refer to Celldex Therapeutics, Inc. and its subsidiaries, except that such terms refer to only Celldex Therapeutics, Inc. and not its subsidiaries in the sections entitled "Description of Common Stock," "Description of Preferred Stock," "Description of Warrants," "Description of Depositary Shares," and "Description of Units."

PROSPECTUS SUMMARY

We are a biopharmaceutical company dedicated to developing therapeutic monoclonal and bispecific antibodies that address diseases for which available treatments are inadequate. Our drug candidates include antibody-based therapeutics which have the ability to engage the human immune system and/or directly affect critical pathways to improve the lives of patients with inflammatory diseases and many forms of cancer.

We are focusing our efforts and resources on the continued research and development of

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Barzolvolimab (also referred to as CDX-0159), a monoclonal antibody that specifically binds the KIT receptor and potently inhibits its activity, which is currently being studied across multiple mast cell driven diseases including

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Chronic Urticarias: In June and July 2022 respectively, we announced that enrollment had opened and the first patients had been dosed in Phase 2 studies in chronic spontaneous urticaria (CSU) and chronic inducible urticaria (CIndU); completion of enrollment to the Phase 2 CSU study was announced in July 2023 and we anticipate reporting topline data from this study in late 2023. Data from the Phase 1b study in CSU were reported in February and June 2023. Positive interim data from the Phase 1b study in CIndU were reported in July and September 2021 and in December 2022 in patients with cold urticaria and symptomatic dermographism. Data from the cholinergic cohort included in the CIndU study were presented in June 2023;

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Prurigo Nodularis (PN): In December 2021 we announced that the first patient had been dosed in a Phase 1b study in PN; enrollment was closed in February 2023 and we plan to present data from the study in November 2023;

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Eosinophilic Esophagitis (EoE): A Phase 2 study in EoE was initiated in June 2023 and the first patient was dosed late that month.

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Our next generation bispecific antibody platform to support pipeline expansion with additional candidates for inflammatory diseases and oncology. Targets are being selected based on new science as well as their compatibility to be used in bispecific antibody formats with our existing antibody programs. Development is focused on emerging, important pathways controlling inflammatory diseases or immunity to tumors.

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Our goal is to build a fully integrated, commercial-stage biopharmaceutical company that develops important therapies for patients with unmet medical needs. We believe our program assets provide us with the strategic options to either retain full economic rights to our innovative therapies or seek favorable economic terms through advantageous commercial partnerships. This approach allows us to maximize the overall value of our technology and product portfolio while best ensuring the expeditious development of each individual product.

Corporate Information

We are a Delaware corporation organized in 1983. Our principal executive offices are located at Perryville III Building, 53 Frontage Road, Suite 220, Hampton, New Jersey 08827 and our telephone number is (908) 200-7500. Our corporate website is www.celldex.com. The information on or that can be accessed through our website is not incorporated by reference into this prospectus.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, including the documents that we incorporate by reference, contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. Any statements about our expectations, beliefs, plans, objectives, assumptions or future events or performance are not historical facts and may be forward-looking. These statements are often, but not always, made through the use of words or phrases such as "anticipate," "estimate," "plans," "projects," "continuing," "ongoing," "expects," "management believes," "we believe," "we intend" and similar words or phrases. Accordingly, these statements involve estimates, assumptions and uncertainties, which could cause actual results to differ materially from those expressed in them. Any forward-looking statements are qualified in their entirety by reference to the risk factors discussed in this prospectus or discussed in documents incorporated by reference in this prospectus.

Forward-looking statements are subject to known and unknown risks and uncertainties, which change over time, and are based on management's expectations and assumptions at the time the statements are made, and are not guarantees of future results. Our actual results may differ materially from those expressed or anticipated in the forward-looking statements for many reasons including the factors described in the section entitled "Risk Factors" in this prospectus and in any risk factors described in a supplement to this prospectus or in other filings.

You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date on which they were made. We undertake no obligation to publicly revise any forward-looking statement to reflect circumstances or events after the date of this prospectus or to reflect the occurrence of unanticipated events. You should, however, review the factors and risks we describe in the reports we file from time to time with the SEC after the date of this prospectus. We undertake no obligation to revise or update the forward-looking statements contained in this prospectus at any time. All forward-looking statements are qualified in their entirety by this cautionary statement.

RISK FACTORS

Investing in our securities involves significant risks. Before making an investment decision, you should carefully consider the risks and other information we include or incorporate by reference in this prospectus and any prospectus supplement. In particular, you should consider the risk factors under the heading "Risk Factors" included in our most recent Annual Report on Form 10-K, as may be revised or supplemented by our subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K, each of which are on file with the SEC and are incorporated herein by reference, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future. The risks and uncertainties we have described are not the only ones facing our company. Additional risks and uncertainties not currently known to us or that we currently deem immaterial may also affect our business operations. Additional risk factors may be included in a prospectus supplement relating to a particular offering of securities. Our business, financial condition or results of operations could be materially adversely affected by any of these risks. The trading price of our securities could decline due to any of these risks, and you may lose all or part of your investment. This prospectus is qualified in its entirety by these risk factors.

USE OF PROCEEDS

Unless otherwise provided in the applicable prospectus supplement to this prospectus used to offer specific securities, we expect to use the net proceeds from any offering of securities by us for general corporate purposes, which may include acquisitions, capital expenditures, investments, payment of milestone payments, and the repayment, redemption or refinancing of all or a portion of any indebtedness or other securities outstanding at a particular time, to fund our operations until we receive FDA approval of our products and are able to commercialize our products and to make substantial investments to establish sales, marketing, quality control, and regulatory compliance capabilities in anticipation of FDA approval of our products. Pending the application of the net proceeds, we expect to invest the net proceeds in short-term, interest-bearing instruments with a maturity of three months or less at the date of purchase and consist primarily of investments in money market mutual funds with commercial banks and financial institutions or other investment-grade securities. Such investments may include depositing such net proceeds into, and maintaining cash balances with, financial institutions in excess of insured limits.

SELLING SECURITYHOLDERS

Information about selling securityholders, if any, will be set forth in a prospectus supplement, in an amendment to the registration statement of which this prospectus is a part, or in other filings we make with the SEC under the Exchange Act, which are incorporated by reference.

DESCRIPTIONS OF SECURITIES WE MAY OFFER

This prospectus contains summary descriptions of the common stock, preferred stock, warrants, depositary shares and units that we may offer and sell from time to time. The preferred stock may also be exchangeable for and/or convertible into shares of common stock or another series of preferred stock. When one or more of these securities are offered in the future, a prospectus supplement will explain the particular terms of the securities and the extent to which these general provisions may apply. These summary descriptions and any summary descriptions in the applicable prospectus supplement do not purport to be complete descriptions of the terms and conditions of each security and are qualified in their entirety by reference to our third restated certificate of incorporation, as amended, our amended and restated by-laws and by applicable Delaware law and any other documents referenced in such summary descriptions and from which such summary descriptions are derived. If any particular terms of a security described in the applicable prospectus supplement differ from any of the terms described herein, then the terms described herein will be deemed superseded by the terms set forth in that prospectus supplement.

We may issue securities in book-entry form through one or more depositaries, such as The Depository Trust Company, Euroclear or Clearstream, named in the applicable prospectus supplement. Each sale of a security in book-entry form will settle in immediately available funds through the applicable depositary, unless otherwise stated. We will issue the securities only in registered form, without coupons, although we may issue the securities in bearer form if so specified in the applicable prospectus supplement. If any securities are to be listed or quoted on a securities exchange or quotation system, the applicable prospectus supplement will say so.

DESCRIPTION OF COMMON STOCK

As of November 3, 2023 we are authorized to issue up to 297,000,000 shares of common stock, $0.001 par value per share. As of October 27, 2023, approximately 47,264,197 shares of common stock were outstanding. All outstanding shares of our common stock are fully paid and non-assessable. Our common stock is listed on the Nasdaq Capital Market under the symbol "CLDX".

Dividends

The Board of Directors may, out of funds legally available, at any regular or special meeting, declare dividends to the holders of shares of our common stock as and when they deem expedient, subject to the rights of holders of the preferred stock, if any.

Voting

Each share of common stock entitles the holders to one vote per share on all matters requiring a vote of the stockholders, including the election of directors. No holders of shares of common stock shall have the right to vote such shares cumulatively in any election for the board of directors.

Rights Upon Liquidation

In the event of our voluntary or involuntary liquidation, dissolution, or winding up, the holders of our common stock will be entitled to share ratably in our assets available for distribution after payment in full of all debts and after the holders of preferred stock, if any, have received their liquidation preferences in full.

Miscellaneous

No holders of shares of our common stock shall have any preemptive rights to subscribe for, purchase or receive any shares of any class, whether now or hereafter authorized, or any options or warrants to purchase any such shares, or any securities convertible into or exchanged for any such shares, which may at any time be issued, sold or offered for sale by Celldex.

Anti-Takeover Provisions

Certain provisions in our third restated certificate of incorporation, as amended, and applicable Delaware corporate law may have the effect of discouraging a change of control of Celldex, even if such a transaction is favored by some of our stockholders and could result in stockholders receiving a substantial premium over the current market price of our shares. The primary purpose of these provisions is to encourage negotiations with our management by persons interested in acquiring control of our corporation. These provisions may also tend to perpetuate present management and make it difficult for stockholders owning less than a majority of the shares to be able to elect even a single director.

Computershare Trust Company, N.A. is presently the transfer agent and registrar for our common stock.

DESCRIPTION OF PREFERRED STOCK

At November 3, 2023, the Company had authorized preferred stock comprised of 3,000,000 shares of Class C Preferred Stock of which 350,000 shares have been designated as Class C-1 Junior Participating Cumulative Preferred Stock, the terms of which are to be determined by our Board of Directors. As of November 3, 2023, there was no preferred stock outstanding.

Class C Preferred Stock

This section describes the general terms and provisions of our Class C Preferred Stock. The applicable prospectus supplement will describe the specific terms of the shares of preferred stock offered through that prospectus supplement, as well as any general terms described in this section that will not apply to those shares of preferred stock.

Our board of directors has been authorized to provide for the issuance of the 2,650,000 unissued and undesignated shares of our Class C Preferred Stock. In general, our third restated certificate of incorporation, as amended, authorizes our board of directors to issue new shares of our common stock or preferred stock without further stockholder action, provided that there are sufficient authorized shares.

With respect to each series of our Class C Preferred Stock, our board of directors has the authority to fix the following terms:

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the designation of the series;

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the number of shares within the series;

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whether dividends are cumulative and, if cumulative, the dates from which dividends are cumulative;

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the rate of any dividends, any conditions upon which dividends are payable, and the dates of payment of dividends;

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whether interests in the shares of preferred stock will be represented by depositary shares as more fully described below under "Description of Depositary Shares";

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whether the shares are redeemable, the redemption price and the terms of redemption;

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the amount payable to you for each share you own if we dissolve or liquidate;

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whether the shares are convertible or exchangeable, the price or rate of conversion or exchange, and the applicable terms and conditions;

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any restrictions on issuance of shares in the same series or any other series;

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voting rights applicable to the series of preferred stock; and

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any other rights, priorities, preferences, restrictions or limitations of such series.

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The rights with respect to any shares of our Class C Preferred Stock will be subordinate to the rights of our general creditors. Shares of our Class C Preferred Stock that we issue in accordance with their terms will be fully paid and nonassessable, and will not be entitled to preemptive rights unless specified in the applicable prospectus supplement.

Our ability to issue preferred stock, or rights to purchase such shares, could discourage an unsolicited acquisition proposal. For example, we could impede a business combination by issuing a series of preferred stock containing class voting rights that would enable the holders of such preferred stock to block a business combination transaction. Alternatively, we could facilitate a business combination transaction by issuing a series of preferred stock having sufficient voting rights to provide a required percentage vote of the stockholders. Additionally, under certain circumstances, our issuance of preferred stock could adversely affect the voting power of the holders of our common stock. Although our board of directors is required to make any determination to issue any preferred stock based on its judgment as to the best interests of our stockholders, our board of directors could act in a manner that would discourage an acquisition attempt or other transaction that some, or a majority, of our stockholders might believe to be in their best interests or in which stockholders might receive a premium for their stock over prevailing market prices of such stock.

Our board of directors does not at present intend to seek stockholder approval prior to any issuance of currently authorized stock, unless otherwise required by law or applicable stock exchange requirements.

Terms of the Preferred Stock That We May Offer and Sell to You

We summarize below some of the provisions that will apply to the preferred stock that we may offer to you unless the applicable prospectus supplement provides otherwise. This summary may not contain all information that is important to you. You should read the prospectus supplement, which will contain additional information and which may update or change some of the information below. Prior to the issuance of a new series of preferred stock, we will further amend our third restated certificate of incorporation, as amended, designating the stock of that series and the terms of that series. We will file a copy of the certificate of designation that contains the terms of each new series of preferred stock with the SEC each time we issue a new series of preferred stock. Each certificate of designation will establish the number of shares included in a designated series and fix the designation, powers, privileges, preferences and rights of the shares of each series as well as any applicable qualifications, limitations or restrictions. You should refer to the applicable certificate of designation as well as our third restated certificate of incorporation, as amended, before deciding to buy shares of our preferred stock as described in the applicable prospectus supplement.

Our board of directors has the authority, without further action by the stockholders, to issue preferred stock in one or more series and to fix the number of shares, dividend rights, conversion rights, voting rights, redemption rights, liquidation preferences, sinking funds, and any other rights, preferences, privileges and restrictions applicable to each such series of preferred stock.

The issuance of any preferred stock could adversely affect the rights of the holders of common stock and, therefore, reduce the value of the common stock. The ability of our board of directors to issue preferred stock could discourage, delay or prevent a takeover or other corporate action.

The terms of any particular series of preferred stock will be described in the prospectus supplement relating to that particular series of preferred stock, including, where applicable:

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the designation, stated value and liquidation preference of such preferred stock;

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the number of shares within the series;

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the offering price;

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the dividend rate or rates (or method of calculation), the date or dates from which dividends shall accrue, and whether such dividends shall be cumulative or noncumulative and, if cumulative, the dates from which dividends shall commence to cumulate;

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whether interests in the shares of preferred stock will be represented by depositary shares as more fully described below under "Description of Depositary Shares";

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any redemption or sinking fund provisions;

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the amount that shares of such series shall be entitled to receive in the event of our liquidation, dissolution or winding-up;

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the terms and conditions, if any, on which shares of such series shall be convertible or exchangeable for shares of our stock of any other class or classes, or other series of the same class;

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the voting rights, if any, of shares of such series;

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the status as to reissuance or sale of shares of such series redeemed, purchased or otherwise reacquired, or surrendered to us on conversion or exchange;

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the conditions and restrictions, if any, on the payment of dividends or on the making of other distributions on, or the purchase, redemption or other acquisition by us or any subsidiary, of the common stock or of any other class of our shares ranking junior to the shares of such series as to dividends or upon liquidation;

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the conditions and restrictions, if any, on the creation of indebtedness by us or by any subsidiary, or on the issuance of any additional stock ranking on a parity with or prior to the shares of such series as to dividends or upon liquidation; and

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any additional dividend, liquidation, redemption, sinking or retirement fund and other rights, preferences, privileges, limitations and restrictions of such preferred stock.

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The description of the terms of a particular series of preferred stock in the applicable prospectus supplement will not be complete. You should refer to the applicable amendment to our third restated certificate of incorporation, as amended, for complete information regarding a series of preferred stock.

The preferred stock will, when issued against payment of the consideration payable therefor, be fully paid and nonassessable. Unless otherwise specified in the applicable prospectus supplement, each series of preferred stock will, upon issuance, rank senior to the common stock and on a parity in all respects with each other outstanding series of preferred stock. The rights of the holders of our preferred stock will be subordinate to that of our general creditors.

DESCRIPTION OF WARRANTS

We summarize below some of the provisions that will apply to the warrants unless the applicable prospectus supplement provides otherwise. This summary may not contain all information that is important to you. The complete terms of the warrants will be contained in the applicable warrant certificate and warrant agreement. These documents have been or will be included or incorporated by reference as exhibits to the registration statement of which this prospectus is a part. You should read the warrant certificate and the warrant agreement. You should also read the prospectus supplement, which will contain additional information and which may update or change some of the information below.

General

We may issue, together with other securities or separately, warrants to purchase common stock, preferred stock or other securities. We may issue the warrants under warrant agreements to be entered into between us and a bank or trust company, as warrant agent, all as set forth in the applicable prospectus supplement. The warrant agent would act solely as our agent in connection with the warrants of the series being offered and would not assume any obligation or relationship of agency or trust for or with any holders or beneficial owners of warrants.

The applicable prospectus supplement will describe the following terms, where applicable, of warrants in respect of which this prospectus is being delivered:

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the title of the warrants;

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the designation, amount and terms of the securities for which the warrants are exercisable and the procedures and conditions relating to the exercise of such warrants;

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the designation and terms of the other securities, if any, with which the warrants are to be issued and the number of warrants issued with each such security;

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the price or prices at which the warrants will be issued;

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the aggregate number of warrants;

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any provisions for adjustment of the number or amount of securities receivable upon exercise of the warrants or the exercise price of the warrants;

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the price or prices at which the securities purchasable upon exercise of the warrants may be purchased;

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if applicable, the date on and after which the warrants and the securities purchasable upon exercise of the warrants will be separately transferable;

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if applicable, a discussion of the material U.S. federal income tax considerations applicable to the warrants;

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any other terms of the warrants, including terms, procedures and limitations relating to the exchange and exercise of the warrants;

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the date on which the right to exercise the warrants shall commence and the date on which the right shall expire;

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if applicable, the maximum or minimum number of warrants which may be exercised at any time;

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the identity of the warrant agent;

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any mandatory or optional redemption provision;

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whether the warrants are to be issued in registered or bearer form;

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whether the warrants are extendible and the period or periods of such extendibility;

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information with respect to book-entry procedures, if any; and

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any other terms of the warrants.

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Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including the right to receive dividends, if any, or payments upon our liquidation, dissolution or winding-up or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder thereof to purchase such number of shares of common stock or preferred stock or other securities at the exercise price as will in each case be set forth in, or be determinable as set forth in, the applicable prospectus supplement. Warrants may be exercised at any time up to the close of business on the expiration date set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void. Warrants may be exercised as set forth in the applicable prospectus supplement relating to the warrants offered thereby. Upon receipt of payment and the warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will, as soon as practicable, forward the purchased securities. If less than all of the warrants represented by the warrant certificate are exercised, a new warrant certificate will be issued for the remaining warrants.

Enforceability of Rights of Holders of Warrants

Each warrant agent will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, that holder's warrant(s).

Modification of the Warrant Agreement

The warrant agreement will permit us and the warrant agent, without the consent of the warrant holders, to supplement or amend the agreement in the following circumstances:

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to cure any ambiguity;

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to correct or supplement any provision which may be defective or inconsistent with any other provisions; or

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to add new provisions regarding matters or questions that we and the warrant agent may deem necessary or desirable and which do not adversely affect the interests of the warrant holders.

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DESCRIPTION OF DEPOSITARY SHARES

We summarize below some of the provisions that will apply to depositary shares unless the applicable prospectus supplement provides otherwise. This summary may not contain all information that is important to you. The complete terms of the depositary shares will be contained in the depositary agreement and depositary receipt applicable to any depositary shares. These documents have been or will be included or incorporated by reference as exhibits to the registration statement of which this prospectus is a part. You should read the depositary agreement and the depositary receipt. You should also read the prospectus supplement, which will contain additional information and which may update or change some of the information below.

General

We may, at our option, elect to offer fractional or multiple shares of common stock or preferred stock, rather than single shares of common stock or preferred stock (to be set forth in the prospectus supplement relating to such depositary shares). In the event we elect to do so, depositary receipts evidencing depositary shares will be issued to the public.

The shares of common stock or any class or series of preferred stock represented by depositary shares will be deposited under a deposit agreement among us, a depositary selected by us, and the holders of the depositary receipts. The depositary will be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50,000,000. Subject to the terms of the deposit agreement, each owner of a depositary share will be entitled, in proportion to the applicable fraction of a share of common stock or preferred stock represented by such depositary share, to all the rights and preferences of the shares of common stock or preferred stock represented by the depositary share, including dividend, voting, redemption and liquidation rights.

The depositary shares will be evidenced by depositary receipts issued pursuant to the deposit agreement. Depositary receipts will be distributed to those persons purchasing the fractional shares of common stock or the related class or series of preferred shares in accordance with the terms of the offering described in the related prospectus supplement.

DESCRIPTION OF UNITS

We may issue units comprised of one or more of the other securities described in this prospectus in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date. The applicable prospectus supplement may describe:

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the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;

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any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units;

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the terms of the unit agreement governing the units;

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United States federal income tax considerations relevant to the units; and

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whether the units will be issued in fully registered global form.

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This summary of certain general terms of units and any summary description of units in the applicable prospectus supplement do not purport to be complete and are qualified in their entirety by reference to all provisions of the applicable unit agreement and, if applicable, collateral arrangements and depositary arrangements relating to such units. The forms of the unit agreements and other documents relating to a particular issue of units will be filed with the SEC each time we issue units, and you should read those documents for provisions that may be important to you.

PLAN OF DISTRIBUTION

We may sell the securities covered hereby from time to time pursuant to underwritten public offerings, direct sales to the public, negotiated transactions, block trades or a combination of these methods. A distribution of the securities offered by this prospectus may also be effected through the issuance of derivative securities, including without limitation, warrants and subscriptions. We may sell the securities to or through underwriters or dealers, through agents, or directly to one or more purchasers. We may distribute securities from time to time in one or more transactions:

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at a fixed price or prices, which may be changed;

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at market prices prevailing at the time of sale;

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at prices related to such prevailing market prices;

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at varying prices determined at the time of sale; or

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at negotiated prices.

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We may also sell equity securities covered by this prospectus in "at the market offerings" as defined in Rule 415(a)(4) under the Securities Act. Such offerings may be made into an existing trading market for such securities in transactions at other than a fixed price on or through the facilities of the Nasdaq Capital Market or any other securities exchange or quotation or trading service on which such securities may be listed, quoted or traded at the time of sale.

A prospectus supplement or supplements will describe the terms of the offering of the securities, including:

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the name or names of the underwriters, dealers or agents participating in the offering, if any;

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the purchase price of the securities sold by us to any underwriter or dealer and the net proceeds we expect to receive from the offering;

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any over-allotment options under which underwriters may purchase additional securities from us;

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any agency fees or underwriting discounts or commissions and other items constituting agents' or underwriters' compensation;

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any public offering price;

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any discounts or concessions allowed or reallowed or paid to dealers; and

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any securities exchange or market on which the securities may be listed.

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Only underwriters named in the prospectus supplement will be underwriters of the securities offered by the prospectus supplement.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any over-allotment option. Any public offering price and any discounts or commissions or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions and other compensation we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to

delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

We may provide agents and underwriters with indemnification against civil liabilities related to this offering, including liabilities under the Securities Act, or contribution with respect to payments that the agents or underwriters may make with respect to these liabilities. Agents and underwriters may engage in transactions with, or perform services for, us in the ordinary course of business.

All securities we may offer, other than common stock, will be new issues of securities with no established trading market. Any agents or underwriters may make a market in these securities, but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities. There is currently no market for any of the offered securities, other than our common stock, which is listed on the Nasdaq Capital Market. We have no current plans for listing of the preferred stock, warrants or subscription rights on any securities exchange or quotation system; any such listing with respect to any particular preferred stock, warrants or subscription rights will be described in the applicable prospectus supplement or other offering materials, as the case may be.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

Any agents and underwriters who are qualified market makers on the Nasdaq Capital Market may engage in passive market making transactions in the securities on the Nasdaq Capital Market in accordance with Regulation M, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the securities. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker's bid, however, the passive market maker's bid must then be lowered when certain purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.

LEGAL MATTERS

Unless otherwise indicated in the applicable prospectus supplement, the validity of the securities offered hereby will be passed upon for us by Lowenstein Sandler LLP, New York, New York. If the validity of the securities offered hereby in connection with offerings made pursuant to this prospectus are passed upon by counsel for the underwriters, dealers or agents, if any, such counsel will be named in the prospectus supplement relating to such offering.

EXPERTS

The financial statements and management's assessment of the effectiveness of internal control over financial reporting (which is included in Management's Annual Report on Internal Control over Financial Reporting) incorporated in this Prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2022 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC a registration statement on Form S-3, including exhibits, under the Securities Act of which this prospectus forms a part. This prospectus does not contain all of the information set forth in the registration statement. This prospectus contains descriptions of certain agreements or documents that are exhibits to the registration statement. The statements as to the contents of such exhibits, however, are brief descriptions and are not necessarily complete, and each statement is qualified in all respects by reference to such agreement or document. For further information about us, please refer to the registration statement and the documents incorporated by reference in this prospectus.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC's website at http://www.sec.gov. The SEC's website contains reports, proxy statements and other information regarding issuers, such as Celldex Therapeutics, Inc., that file electronically with the SEC. We make available free of charge through our web site our Annual Report on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, Proxy Statements on Schedule 14A and all amendments to those reports as soon as reasonably practicable after such material is electronically filed with or furnished to the SEC. Our website address is http://www.celldextherapeutics.com. Please note that our website address is provided as an inactive textual reference only. Information contained on or accessible through our website is not part of this prospectus or the prospectus supplement, and is therefore not incorporated by reference unless such information is otherwise specifically referenced elsewhere in this prospectus or the prospectus supplement.

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to "incorporate by reference" into this prospectus the information we have filed with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information that we file subsequently with the SEC will automatically update this prospectus. We incorporate by reference into this prospectus the information contained in the documents listed below, which is considered to be a part of this prospectus:

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our Annual Report on Form 10-K for the year ended December 31, 2022, filed with the SEC on February 28, 2023;

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the information in our Definitive Proxy Statement on Schedule 14A, filed with the SEC on April 25, 2023, but only to the extent incorporated by reference into our Annual Report on Form 10-K for the year ended December 31, 2022;

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our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2023, June 30, 2023 and September 30, 2023, filed with the SEC on May 4, 2023, August 8, 2023 and November 2, 2023, respectively;

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our Current Reports on Form 8-K filed with the SEC on February 21, 2023, March 22, 2023 and June 15, 2023 (in each case, other than any portions thereof deemed furnished and not filed); and

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The description of our common stock contained in our Registration Statement on Form 8-A, filed on November 8, 2004, as amended by Form 8-A/A filed on October 22, 2007 and March 7, 2008.

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We also incorporate by reference all documents we file under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (a) after the initial filing date of the registration statement of which this prospectus is a part and before the effectiveness of the registration statement and (b) after the effectiveness of the registration statement and before the filing of a post-effective amendment that indicates that the securities offered by this prospectus have been sold or that deregisters the securities covered by this prospectus then remaining unsold. The most recent information that we file with the SEC automatically updates and supersedes older information. The information contained in any such filing will be deemed to be a part of this prospectus, commencing on the date on which the document is filed.

Nothing in this prospectus shall be deemed to incorporate information furnished but not filed with the SEC pursuant to Item 2.02 or 7.01 of Form 8-K.

We will furnish without charge to each person, including any beneficial owner, to whom this prospectus is delivered, upon written or oral request, a copy of any documents incorporated by reference other than exhibits to those documents. Requests should be addressed to:

Corporate Secretary
Celldex Therapeutics, Inc.
Perryville III Building, 53 Frontage Road, Suite 220,
Hampton, New Jersey 08827
(908) 200-7500

Any statement contained in this prospectus or in a document incorporated or deemed to be incorporated by reference into this prospectus will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or any other subsequently filed document that is deemed to be incorporated by reference into this prospectus modifies or supersedes the statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

You should rely only on information contained in, or incorporated by reference into, this prospectus. We have not authorized anyone to provide you with information different from that contained in this prospectus or incorporated by reference in this prospectus. We are not making offers to sell the securities in any jurisdiction in which such an offer or solicitation is not authorized or in which the person making such offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make such offer or solicitation.

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10,345,000 Shares of Common Stock

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PROSPECTUS SUPPLEMENT​

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Joint Bookrunning Managers

Co-Lead Managers

April 1, 2026

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