Post-Effective Amendment to Registration Statement (Form POS AMI)

As filed with the Securities and Exchange Commission on March 13, 2026

Investment Company Act File No. 811-08767

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 [X]

Amendment No. 103

UBS Series Funds

(Exact Name of Registrant Specified in Charter)

c/o UBS Asset Management (Americas) LLC

1285 Avenue of the Americas

New York, New York 10019

(Address of Principal Executive Offices)

Registrant's telephone number, including area code: (888) 793-8637

KEITH A. WELLER, ESQ.

UBS ASSET MANAGEMENT (AMERICAS) LLC

One North Wacker Drive

Chicago, Illinois 60606

(Name and address of agent for service)

Copies to:

STEPHEN H. BIER, ESQ.

DECHERT LLP

Three Bryant Park

1095 Avenue of the Americas

New York, New York 10036

Telephone: (212) 698-3500

It is intended that this filing will become effective immediately upon filing in accordance with Section 8 of the Investment Company Act of 1940 and rules thereunder.

Explanatory Note

UBS Series Funds (the "Trust") has filed this Amendment No. 103 to the Registration Statement of the Trust on Form N-1A (File No. 811-08767) (the "Registration Statement") pursuant to Section 8(b) of the Investment Company Act of 1940, as amended (the "1940 Act"). Shares of beneficial interest in Limited Purpose Cash Investment Fund (the "fund"), a series of the Trust, are not being registered under the Securities Act of 1933, as amended (the "1933 Act"), since such shares will be issued solely in private placement transactions that do not involve any "public offering" within the meaning of Section 4(a) (2) of the 1933 Act. Only investment companies, insurance company separate accounts, common or commingled trust funds or other organizations, entities or investors that are "accredited investors" within the meaning of Regulation D under the 1933 Act may make investments in the shares of the fund. Such investors are referred to herein as "shareholders." This Registration Statement is not an offer to sell, or the solicitation of an offer to buy, any shares in the fund.

This Amendment No. 103 to the Registration Statement is being filed under the 1940 Act to amend and supplement Amendment No. 100 to the Registration Statement under the 1940 Act, filed with the U.S. Securities and Exchange Commission ("Commission") on November 14, 2025 (Accession No. 0001193125-25-282826) ("Amendment 100"), as pertaining to Parts A and B of the Registration Statement with respect to the fund. Parts A and B of the Registration Statement with respect to the fund, as filed in Amendment 100, are incorporated by reference herein.

March 13, 2026

Supplement to the prospectuses (each, a "Prospectus" and together, the "Prospectuses") and Part Bs referenced on the reverse, as may have been supplemented.

Includes:

Dear Investor,

The purpose of this supplement is to update certain information in the Prospectuses and Part Bs, as applicable, for the above-named investment trusts, including each series thereof referenced on the reverse (collectively, the "funds"), as described below.

The funds, indicated on the reverse chart, will be closed for business on April 3, 2026, in observance of Good Friday, in accordance with the New York Stock Exchange holiday schedule.

PLEASE BE SURE TO RETAIN THIS IMPORTANT INFORMATION FOR FUTURE REFERENCE.

© UBS 2026. All rights reserved.

UBS Asset Management (Americas) LLC

ZS-1296

This supplement relates to each of the following Prospectuses/Part Bs:

2

PART C

Item 28. Exhibits

60

61

62

63

64

65

Section 2 of Article IX of the Trust Instrument, as amended ("Trust Instrument"), "Indemnification," provides that the appropriate series of the Registrant will indemnify the trustees and officers of the Registrant to the fullest extent permitted by law against claims and expenses asserted against or incurred by them by virtue of being or having been a trustee or officer; provided that no such person shall be indemnified where there has been an adjudication or other determination, as described in Article IX, that such person is liable to the Registrant or its shareholders by reason of willful misfeasance, bad faith, gross negligence or reckless disregard of the duties involved in the conduct of his or her office, or did not act in good faith in the reasonable belief that his action was in the best interest of the Registrant. Section 2 of Article IX also provides that the Registrant may maintain insurance policies covering such rights of indemnification.

Additionally, "Limitation of Liability" in Section 1 of Article IX of the Trust Instrument provides that the trustees or officers of the Registrant shall not be personally liable to any person extending credit to, contracting with or having a claim against the Registrant or a particular series; and that, provided they have exercised reasonable care and have acted under the reasonable belief that their actions are in the best interest of the Registrant, the trustees and officers shall not be liable for neglect or wrongdoing by them or by any officer, agent, employee, investment advisor or independent contractor of the Registrant.

Section 9 (except as otherwise noted) of each (i) Investment Advisory and Administration Contract with respect to UBS Liquid Assets Government Fund; (ii) Section 8 of the Administration Contract with respect to UBS Select Treasury Institutional Fund, UBS Select Treasury Preferred Fund, UBS Prime Reserves Fund, UBS Prime Preferred Fund, UBS Select Government Institutional Fund, UBS Select Government Preferred Fund, UBS Select 100% US Treasury Institutional Fund, UBS Select 100% US Treasury Preferred Fund, and Cantor Fitzgerald Government Money Market Fund; (iii) Section 8 of the Administration Contract with respect to UBS RMA Government Money Market Fund; (iv) Management Contract for Limited Purpose Cash Investment Fund; and (v) Section 11 of the Investment Advisory and Administration Contract with respect to UBS Ultra Short Income Fund (each, an "Advisory/Administration Contract"), with UBS Asset Management (Americas) LLC (formerly, UBS Asset Management (Americas) Inc. and UBS Global Asset Management (Americas) Inc.) ("UBS AM") provides that UBS AM shall not be liable for any error of judgment or mistake of law or for any loss suffered by any series ("Fund") of the Registrant in connection with the matters to which the Advisory/Administration Contract relates, except for a loss resulting from the willful misfeasance, bad faith, or gross negligence of UBS AM in the performance of its duties or from its reckless disregard of its obligations and duties under the Advisory/Administration Contract. Section 10, 11, or 12 of each Advisory/Administration Contract provides that the Trustees shall not be liable for any obligations of the Trust or any series under the Advisory/Administration Contract and that UBS AM shall look only to the assets and property of the Registrant in settlement of such right or claim and not to the assets and property of the trustees.

Section 9 of each Principal Underwriting Contract or Distribution Contract provides that the Trust will indemnify UBS Asset Management (US) Inc. (formerly, UBS Global Asset Management (US) Inc.) ("UBS AM (US)") and its officers, directors and controlling persons against all liabilities arising from any alleged untrue statement of material fact in the Registration Statement or from any alleged omission to state in the Registration Statement a material fact required to be stated in it or necessary to make the statements in it, in light of the circumstances under which they were made, not misleading, except insofar as liability arises from untrue statements or omissions made in reliance upon and in conformity with information furnished by UBS AM (US) to the Trust for use in the Registration Statement; and provided that this indemnity agreement shall not protect any such persons against liabilities arising by reason of their bad faith, gross negligence or willful misfeasance; and shall not inure to the benefit of any such persons unless a court of competent jurisdiction or controlling precedent determines that such result is not against public policy as expressed in the Securities Act of 1933, as amended (the "1933 Act"). Section 9 of each Principal Underwriting Contract or Distribution Contract also provides that UBS AM (US) agrees to indemnify, defend and hold the Trust, its officers and trustees free and harmless of any claims arising out of any alleged untrue statement or any alleged omission of material fact contained in information furnished by UBS AM (US) for use in the Registration Statement or arising out of an agreement between UBS AM (US) and any retail dealer, or arising out of supplementary literature or advertising used by UBS AM (US) in connection with the Contract.

66

Section 15 or 16 of each Principal Underwriting Contract and Section 10 of the Distribution Contract contain provisions similar to Section 10, 11, or 12 of the Advisory/Administration Contracts, with respect to UBS AM (US).

Section 9, 14, or 15 of each Dealer Agreement, and Section 12 of the Mutual Fund Account Administration Agreement, contains provisions similar to those of Section 9 of the Principal Underwriting Contract or Distribution Contract with respect to the applicable dealer.

The Exclusive Placement Agent Agreement contains provisions similar to those of Section 9 of the Principal Underwriting Contract or Distribution Contract with respect to the applicable dealer.

Insofar as indemnification for liabilities arising under the 1933 Act may be provided to trustees, officers and controlling persons of the Registrant, pursuant to the foregoing provisions or otherwise, the Registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the 1933 Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a trustee, officer or controlling person of the Registrant in connection with the successful defense of any action, suit or proceeding or payment pursuant to any insurance policy) is asserted against the Registrant by such trustee, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the 1933 Act and will be governed by the final adjudication of such issue.

Item 31. Business and Other Connections of Investment Advisor

UBS AM, a Delaware limited liability company, is a registered investment advisor and is an indirect wholly owned subsidiary of UBS Group AG. UBS AM is primarily engaged in the investment advisory and financial services business. Set forth below in alphabetical order is a list of certain executive officers and each board manager of UBS AM indicating each business, profession, vocation or employment of a substantial nature in which each such person has been engaged during the last two fiscal years. (While each board manager is named below, the list of executive officers has been shortened as the full list would be very long and contain names of persons whose functions are unrelated to the Registrant.)

67

Messrs. Mullen, Sanders, Stacey and Weller and Ms. Merrill are employed by UBS Business Solutions US LLC.

Item 32. Principal Underwriter/Placement Agent

(a)  UBS AM (US) serves as principal underwriter or placement agent for the following other investment companies:

CREDIT SUISSE COMMODITY STRATEGY FUNDS

CREDIT SUISSE OPPORTUNITY FUNDS

CREDIT SUISSE TRUST

CREDIT SUISSE ASSET MANAGEMENT INCOME FUND, INC.

CREDIT SUISSE HIGH YIELD BOND FUND

MASTER TRUST

PACE SELECT ADVISORS TRUST

SMA RELATIONSHIP TRUST

THE UBS FUNDS

UBS INVESTMENT TRUST

(b)  UBS AM (US) is the Registrant's principal underwriter or placement agent. The directors and certain principal executive officers of UBS AM (US), their principal business addresses, and their positions and offices with UBS AM (US), are identified below along with those directors and officers of UBS AM (US) who also serve as trustees or officers of the Registrant. (While each board director is named below, the list of executive officers has been shortened as the full list would be very long and contain names of persons whose functions are unrelated to the Registrant.)

68

* This person's business address is 1285 Avenue of the Americas, New York, NY 10019.

** This person's business address is One North Wacker Drive, Chicago, IL 60606.

*** This person's business address is 1000 Harbor Boulevard, Weehawken, NJ 07086.

**** This person's business address is 555 California Street, 36th Floor, San Francisco, CA 94104.

***** This person's business address is 11 Madison Avenue, New York, NY 10010.

(c)  None.

Item 33. Location of Accounts and Records

The books and other documents required (i) by paragraphs (b)(4), (c) and (d) of Rule 31a-1 and (ii) by paragraphs (a)(3), (a)(4), (a)(5), (c) and (e) of Rule 31a-2 under the Investment Company Act of 1940 are maintained in the physical possession of UBS AM, at 1285 Avenue of the Americas, New York, NY 10019, 1000 Harbor Boulevard,

69

Weehawken, NJ 07086, and One North Wacker Drive, Chicago, IL 60606. Certain information required by Rule 31a-1(b)(1) to be maintained by a money market fund is maintained in the possession of UBS AM, at 1285 Avenue of the Americas, New York, NY 10019, 1000 Harbor Boulevard, Weehawken, NJ 07086, and One North Wacker Drive, Chicago, IL 60606. All other accounts, books and documents required by Rule 31a-1 are maintained in the physical possession of Registrant's transfer agent and custodian.

Item 34. Management Services

Not applicable.

Item 35. Undertakings

Not applicable.

70

SIGNATURES

Pursuant to the requirements of the Investment Company Act of 1940, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Chicago and State of Illinois, on the 13th day of March, 2026.