Exhibit 5.1

SIMPSON THACHER & BARTLETT LLP

2475 HANOVER STREET

PALO ALTO, CA 94304

(650) 251-5000

FACSIMILE (650) 251-5002

December 10, 2021

Microsoft Corporation

One Microsoft Way

Redmond, Washington 98052-6399

Ladies and Gentlemen:

We have acted as counsel to Microsoft Corporation, a Washington corporation (the “Company”), in connection with the Registration Statement on Form S-3 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended, relating to the issuance by the Company from time to time of an indeterminate aggregate initial offering price of senior unsecured debt securities (the “Securities”). The Securities will be issued under an Indenture dated as of May 18, 2009 (the “Indenture”), between the Company and The Bank of New York Mellon Trust Company, N.A., as trustee.

We have examined the Registration Statement and the Indenture (including the form of Securities set forth therein), which is an exhibit to the Registration Statement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and such certificates or comparable documents of public officials and of officers and representatives of the Company and have made such other investigations as we have deemed relevant and necessary in connection with the opinion hereinafter set forth.

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents. We also have assumed that at the time of the execution, authentication and delivery of the Securities, the Indenture will be the valid and legally binding obligation of the applicable Trustees.

Microsoft Corporation - 2 - December 10, 2021

In rendering the opinion set forth below, we have assumed further that the execution, issuance, delivery and performance by the Company of the Indenture (a) do not constitute a breach or default under any agreement or instrument which is binding upon the Company and (b) comply with all applicable regulatory requirements.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that assuming (a) the taking of all necessary corporate action by the Board of Directors of the Company, a duly constituted and acting committee of such Board or duly authorized officers of the Company (such Board of Directors, committee or authorized officers being hereinafter referred to as the “Board”) to authorize and approve the issuance and terms of the Securities and the terms of the offering thereof so as to not violate any applicable law or agreement or instrument then binding on the Company and (b) the due execution, authentication, issuance and delivery of the Securities, upon payment therefor in accordance with the applicable definitive underwriting, purchase or similar agreement approved by the Board and otherwise in accordance with the provisions of the Indenture and such agreement, the Securities will constitute valid and legally binding obligations of the Company enforceable against the Company in accordance with their terms.

Our opinion set forth above is subject to (i) the effects of bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally, (ii) general equitable principles (whether considered in a proceeding in equity or at law) and (iii) an implied covenant of good faith and fair dealing. In addition, we express no opinion as to the validity, legally binding effect or enforceability of (i) the waiver of rights and defenses contained in Section 114 of the Indenture or (ii) Section 110 of the Indenture relating to the separability of the provisions of the Indenture.

Insofar as the opinion expressed herein relate to or are dependent upon matters governed by the law of the State of Washington, we have relied, without any independent investigation, upon the opinion of Keith R. Dolliver, Esq., Vice President and Deputy General Counsel of the Company dated the date hereof and our opinion is subject to the qualifications, assumptions, limitations and exceptions set forth therein.

We do not express any opinion herein concerning any law other than the law of the State of New York and, to the extent set forth herein, the law of the State of Washington.

Microsoft Corporation - 3 - December 10, 2021

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Validity of the Securities” in the Prospectus included in the Registration Statement.

Very truly yours,

/s/ Simpson Thacher & Bartlett LLP

SIMPSON THACHER & BARTLETT LLP