EX-5.1

Exhibit 5.1

LOGO

Pillsbury Winthrop Shaw Pittman LLP

April 19, 2022

Acer Therapeutics Inc.

One Gateway Center, Suite 351

300 Washington Street

Newton, MA 02458

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

We are acting as counsel for Acer Therapeutics Inc., a Delaware 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”) for purposes of registering under the Securities Act of 1933 (the “Act”) the resale of 2,478,000 shares (the “Shares”) of the Company’s common stock, $0.0001 par value per share, which are reserved for issuance upon conversion of the Company’s secured convertible notes dated March 14, 2022.

We have reviewed and are familiar with such corporate proceedings and other matters as we have considered relevant or necessary for the opinions expressed in this letter. Based upon the foregoing, we are of the opinion that the Shares have been duly authorized and, when issued and delivered by the Company in the manner described in the Registration Statement and the prospectus included therein and in accordance with the resolutions adopted by the Board of Directors of the Company, or a committee thereof, will be validly issued, fully paid and non-assessable.

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 “Legal Matters” in the Registration Statement and in the prospectus included therein. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Pillsbury Winthrop Shaw Pittman LLP