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

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

Investment Company Act File No. 811-22078

U.S. SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

Master Trust

(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

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.

The following funds are series of Master Trust ("Trust"), a professionally managed open-end investment company.

Treasury Master Fund

Prime CNAV Master Fund

Government Master Fund

100% US Treasury Master Fund

Explanatory Note

The Trust has filed this Amendment No. 69 to the Registration Statement of the Trust on Form N-1A (File No. 811-22078) (the "Registration Statement") pursuant to Section 8(b) of the Investment Company Act of 1940, as amended (the "1940 Act"). Beneficial interests in the Trust ("Interests") are not being registered under the Securities Act of 1933, as amended (the "1933 Act"), since such interests 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 Trust. Such investors are referred to herein as "interestholders." This Registration Statement is not an offer to sell, or the solicitation of an offer to buy, any Interests in the Trust.

This Amendment No. 69 to the Registration Statement is being filed under the 1940 Act to amend and supplement Amendment No. 68 to the Registration Statement under the 1940 Act, filed with the U.S. Securities and Exchange Commission ("Commission") on August 22, 2025 (Accession No. 0001193125-25-185451) ("Amendment 68"), as pertaining to Parts A and B of the Registration Statement with respect Treasury Master Fund, Prime CNAV Master Fund, Government Master Fund and 100% US Treasury Master Fund. Parts A and B of the Registration Statement with respect to Treasury Master Fund, Prime CNAV Master Fund, Government Master Fund and 100% US Treasury Master Fund, as filed in Amendment 68, 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:

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PART C

Item 28. Exhibits.

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Item 29. Persons Controlled by or under Common Control with Registrant.

None.

Item 30. Indemnification.

Section 2 of Article IX of the Trust Instrument ("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 interestholders 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.

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Section 9 of the Management Contract with respect to Government Master Fund, Prime CNAV Master Fund, and Treasury Master Fund and Section 10 of the Management Contract with respect to 100% US Treasury Master Fund (the "Management Contracts"), with UBS Asset Management (Americas) LLC ("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 Management Contracts relate, 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 Management Contracts. Section 11 of the Management Contracts provides that the Trustees shall not be liable for any obligations of the Trust or any series under the Management Contracts 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.

The Exclusive Placement Agency Agreement provides that the Trust will indemnify UBS Asset Management (US) Inc. ("UBS AM (US)") and its officers, directors and controlling persons against any losses, claims, damages, liabilities or expenses arising from (1) 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; or (2) the Trust's material breach of a representation, warranty, covenant or agreement contained in the Exclusive Placement Agency Agreement; provided that this indemnity shall not protect any such persons against liabilities arising by reason of their bad faith, gross negligence or willful misfeasance or by the reckless disregard of their obligations and duties under the Exclusive Placement Agency Agreement. The Exclusive Placement Agency Agreement 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 Agreement.

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.)

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

Item 32. Principal Underwriter.

(a) UBS AM (US) is the placement agent for the Registrant and its series, Government Master Fund, Prime CNAV Master Fund, Treasury Master Fund, and 100% US Treasury Master Fund. UBS AM (US) serves as principal underwriter or placement agent for the following other investment companies:

SMA RELATIONSHIP TRUST

PACE SELECT ADVISORS TRUST

THE UBS FUNDS

UBS INVESTMENT TRUST

UBS SERIES FUNDS

CREDIT SUISSE COMMODITY STRATEGY FUNDS

CREDIT SUISSE OPPORTUNITY FUNDS

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CREDIT SUISSE TRUST

CREDIT SUISSE ASSET MANAGEMENT INCOME FUND, INC.

CREDIT SUISSE HIGH YIELD BOND FUND

(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.)

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* This person's business address is 787 Seventh Avenue, 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, 787 Seventh Avenue, New York, NY 10019, 1000 Harbor Boulevard, 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, 787 Seventh Avenue, 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.

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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.