Exhibit 5.1

Wilson Sonsini Goodrich & Rosati
Professional Corporation 650 Page Mill Road
Palo Alto, California 94304-1050 O: 650.493.9300
F: 650.493.6811

March 25, 2022

ALX Oncology Holdings Inc.

323 Allerton Avenue

South San Francisco, California 94080

Re: ALX Oncology Holdings Inc. – Registration Statement on Form S-3

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-3 (the “Registration Statement”) that ALX Oncology Holdings Inc. (the “Company”) is filing with the Securities and Exchange Commission (the “Commission”) on March 25, 2022 for the purpose of registering under the Securities Act of 1933, as amended (the “Securities Act”), the Securities (as defined below).

The Registration Statement relates to the proposed issuance and sale by the Company, from time to time, pursuant to Rule 415 under the Securities Act, as set forth in the Registration Statement, the prospectus contained therein (the “Prospectus”) and the supplements to the prospectus referred to therein (each a “Prospectus Supplement”), of up to an aggregate offering price of $450,000,000 of (i) shares of the Company’s common stock, $0.001 par value per share (the “Common Stock”); (ii) shares of the Company’s preferred stock, $0.001 par value per share (the “Preferred Stock”); (iii) warrants of the Company (the “Warrants”), which may be issued pursuant to a warrant agreement (the “Warrant Agreement”) between the Company and the warrant agent to be named therein (the “Warrant Agent”); (iv) purchase contracts (the “Purchase Contracts”), which may be issued under one or more purchase contract agreements (each such Purchase Contract, a “Purchase Contract Agreement”) to be entered into between the Company and the purchase contract agent to be named therein (the “Purchase Contract Agent”); and (v) units (the “Units”) to be issued under one or more unit agreements to be entered into among the Company, a bank or trust company, as unit agent (the “Unit Agent”), and the holders from time to time of the Units (each such unit agreement, a “Unit Agreement”) (the Common Stock, the Preferred Stock, the Warrants, the Purchase Contracts and the Units are collectively referred to herein as the “Securities”).

We advise you that we have also examined the prospectus supplement to the Registration Statement in connection with the registration under the Securities Act of up to $150,000,000 in Common Stock (the “Shares”) to be issued and sold by the Company. We understand that the Company has agreed to issue and sell the Shares from time to time through Cantor Fitzgerald & Co. and Credit Suisse Securities (USA) LLC, as sales agents (the “Sales Agents”) pursuant to a sales agreement by and among the Company and the Sales Agents (the “Sales Agreement”).

AUSTIN        BEIJING        BOSTON         BRUSSELS        HONG KONG        LONDON        LOS ANGELES        NEW YORK        PALO ALTO

SAN DIEGO        SAN FRANCISCO        SEATTLE        SHANGHAI        WASHINGTON, DC        WILMINGTON, DE

ALX Oncology Holdings Inc.

March 25, 2022

Page 2

We are acting as counsel for the Company in connection with the issuance and sale of the Securities and Shares by the Company. In such capacity, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion. In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity with the originals of all documents submitted to us as copies, the authenticity of the originals of such documents and the legal competence of all signatories to such documents.

We express no opinion herein as to the laws of any state or jurisdiction other than the General Corporation Law of the State of Delaware (including the statutory provisions and all applicable judicial decisions interpreting those laws) and the federal laws of the United States of America.

Based upon the foregoing, we advise you that, in our opinion:

1. When the necessary corporate action on the part of the Company has been taken to authorize the issuance and sale of such shares of Common Stock proposed to be sold by the Company, and when such shares of Common Stock are issued and delivered in accordance with the applicable underwriting or other agreement against payment therefor (in excess of par value thereof) or upon conversion or exercise of any security offered under the Registration Statement (the “Offered Security”), in accordance with terms of such Offered Security or the instrument governing such Offered Security providing for such conversion or exercise as approved by the Board of Directors of the Company (the “Board”), for the consideration approved by the Board (which consideration is not less than the par value of the Common Stock), such shares of Common Stock will be validly issued, fully-paid and non-assessable.

2. Upon designation of the relative rights, preferences and limitations of any series of Preferred Stock by the Board and the proper filing with the Secretary of State of the State of Delaware of a Certificate of Designation relating to such series of Preferred Stock, all necessary corporate action on the part of the Company will have been taken to authorize the issuance and sale of such series of Preferred Stock proposed to be sold by the Company, and when such shares of Preferred Stock are issued and delivered in accordance with the applicable underwriting or other agreement against payment therefor (in excess of par value thereof), such shares of Preferred Stock will be validly issued, fully paid and non-assessable.

3. When the Warrant Agreement to be entered into in connection with the issuance of any Warrants has been duly authorized, executed and delivered by the Warrant Agent and the Company, the specific terms of the Warrants have been duly authorized and established in accordance with the Warrant Agreement and such Warrants have been duly authorized, executed, issued and delivered in accordance with the Warrant Agreement and the applicable underwriting

ALX Oncology Holdings Inc.

March 25, 2022

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or other agreement against payment therefor, such Warrants will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

4. When the Purchase Contract Agreement to be entered into in connection with the issuance of any Purchase Contracts has been duly authorized, executed and delivered by the Purchase Contract Agent and the Company, the specific terms of the Purchase Contracts have been duly authorized and established in accordance with the Purchase Contract Agreement and such Purchase Contracts have been duly authorized, executed, issued and delivered in accordance with the Purchase Contract Agreement and the applicable underwriting or other agreement against payment therefor, such Purchase Contracts will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

5. When the Unit Agreement to be entered into in connection with the issuance of any Units has been duly authorized, executed and delivered by the Unit Agent and the Company, the specific terms of the Units have been duly authorized and established in accordance with the Unit Agreement and such Units have been duly authorized, executed, issued and delivered in accordance with the Unit Agreement and the applicable underwriting or other agreement against payment therefor, such Units will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally, concepts of reasonableness and equitable principles of general applicability.

6. The Shares to be issued and sold by the Company have been duly authorized and, when such Shares are issued and paid for in accordance with the terms of the Sales Agreement, will be validly issued, fully paid and nonassessable.

In connection with the opinions expressed above, we have assumed that, at or prior to the time of the delivery of any such security, (i) the Board shall have duly established the terms of such security and duly authorized the issuance and sale of such security and such authorization shall not have been modified or rescinded; (ii) the Company shall remain validly existing as a corporation in good standing under the laws of the State of Delaware; (iii) the Registration Statement shall have become effective and such effectiveness shall not have been terminated or rescinded; (iv) that the Prospectus Supplement will have been filed with the Commission describing the Securities offered thereby; (v) the Purchase Contract Agreement, the Purchase Agreements, the Unit Agreement, the Units, the Warrant Agreement, and the Warrants, are each valid, binding and enforceable agreements of each party thereto (other than as expressly covered above in respect of the Company); (vi) there shall not have occurred any change in law affecting the validity or enforceability of such Security; (vii) that any Securities issuable upon conversion, exchange, redemption, or exercise of any Securities being offered will be and duly authorized, created and, if appropriate, reserved for issuance upon such conversion, exchange, redemption, or exercise; (viii)

ALX Oncology Holdings Inc.

March 25, 2022

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with respect to shares of Common Stock or Preferred Stock, that there will be sufficient shares of Common Stock or Preferred Stock authorized under the Company’s organizational documents that are not otherwise reserved for issuance; and (ix) the legal capacity of all natural persons. We have also assumed that none of the terms of any security to be established subsequent to the date hereof, the issuance and delivery of such security or the compliance by the Company with the terms of such security will violate any applicable law or public policy or will result in a violation of any provision of any instrument or agreement then binding upon the Company, or any restriction imposed by any court or governmental body having jurisdiction over the Company.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement referred to above, for incorporation by reference into the Registration Statement, and to the use of our name wherever it appears in the Registration Statement, and in any amendment or supplement thereto. In giving such consent, we do not believe that we are “experts” within the meaning of such term as used in the Securities Act or the rules and regulations of the Commission issued thereunder with respect to any part of the Registration Statement, including this opinion as an exhibit or otherwise.

Very truly yours,

/s/ Wilson Sonsini Goodrich & Rosati P.C.

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation