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

August 12, 2022

Apexigen, Inc.

75 Shoreway Road, Suite C,

San Carlos, CA 94070

Re: Registration Statement on Form S-1

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-1 (the “Registration Statement”) filed by Apexigen, Inc., a Delaware corporation (the “Company”) with the Securities and Exchange Commission (the “Commission”) in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of up to 17,316,667 shares of the Company’s Common Stock, $0.0001 par value per share (the “Shares”), to be offered by Lincoln Park Capital Fund, LLC (“Lincoln Park” or the “Selling Stockholder”). We understand that the Shares being offered by the Selling Stockholder have been or may be issued pursuant to the Purchase Agreement dated March 17, 2022, between the Company and Lincoln Park (the “Purchase Agreement”).

We are acting as counsel for the Company in connection with the sale of the Shares by the Company to Lincoln Park pursuant to the Purchase Agreement. As such counsel, we have made legal and factual examinations and inquiries as we have deemed necessary or advisable for the purpose of rendering the opinion set forth below. In rendering the opinion expressed below, 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 addition, we have reviewed originals or copies of such corporate records of the Company, certificates of public officials, a certificate of an officer of the Company as to factual matters, and such other documents that we considered necessary or advisable for the purpose of rendering the opinions set forth below. We have not independently established the facts stated therein.

In our examination, we have assumed the genuineness of all signatures, the authenticity and completeness 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 have also assumed the authority of such persons signing on behalf of the parties thereto other than the Company and the due authorization, execution, and delivery of all documents by the parties thereto other than the Company. We have assumed that the certificates representing the Shares have been properly authenticated by the signature of an authorized officer of the Company’s transfer agent. We have also assumed the conformity of the documents filed with the Commission via the Electronic Data Gathering, Analysis and Retrieval System (“EDGAR”), except for required EDGAR formatting changes, to physical copies submitted for our examination and the absence of any evidence extrinsic to the provisions of the written agreements between the parties that the parties intended a meaning contrary to that expressed by those provisions.

We express no opinion as to any matter relating to the laws of any jurisdiction other than the federal laws of the United States of America and the General Corporation Law of the State of Delaware.

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

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

Apexigen, Inc.

August 12, 2022

Page 2

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set out below, we are of the opinion that the Shares to be issued and sold by the Company, consisting of (i) 150,000 Shares that are currently issued and outstanding have been legally and validly issued, fully paid and non-assessable and (ii) 17,166,667 Shares that may be sold to Lincoln Park under the Purchase Agreement have been duly authorized and, when such Shares are issued and paid for in accordance with the terms of the Purchase Agreement, will be validly issued, fully paid and nonassessable.

This opinion is furnished to you in connection with the filing of the Registration Statement, and is not to be used, circulated, quoted, or otherwise relied upon for any other purpose.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement, and to the use of our name wherever it appears in the Registration Statement, the prospectus forming part of the Registration Statement, any prospectus supplement, and in any amendment or supplement thereto. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

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

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation