As filed with the Securities and Exchange Commission on March 29, 2019

Registration No. 333-        

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-8

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

SUTRO BIOPHARMA, INC.

(Exact Name of Registrant as Specified in Its Charter)

310 Utah Avenue, Suite 150

South San Francisco, California, 94080

(650) 392-8412

(Address of Principal Executive Offices) (Zip Code)

2018 Equity Incentive Plan

2018 Employee Stock Purchase Plan

(Full Title of the Plans)

William J. Newell

Chief Executive Officer

Sutro Biopharma, Inc.

310 Utah Avenue, Suite 150

South San Francisco, CA 94080

(Name and Address of Agent for Service)

(650) 392-8412

(Telephone Number, including area code, of agent for service)

Please send copies of all communications to:

Effie Toshav, Esq.

Robert A. Freedman, Esq.

Amanda L. Rose, Esq.

Fenwick & West LLP

555 California Street, 12th Floor

San Francisco, California 94104

(415) 875-2300

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

CALCULATION OF REGISTRATION FEE

REGISTRATION OF ADDITIONAL SHARES

PURSUANT TO GENERAL INSTRUCTION E

Sutro Biopharma, Inc. (the “Registrant”) is filing this Registration Statement with the Securities and Exchange Commission (the “Commission”) to register (a) 1,142,409 additional shares of common stock available for issuance under the Registrant’s 2018 EIP, pursuant to the provision of the 2018 EIP providing for an annual 5% automatic increase in the number of shares reserved for issuance and (b) 228,481 additional shares of common stock available for issuance under the Registrant 2018 ESPP, pursuant to the provision of the 2018 ESPP providing for an annual 1% automatic increase in the number of shares reserved for issuance.

In accordance with General Instruction E of Form S-8, and only with respect to the common stock issuable under the 2018 EIP and 2018 ESPP, this Registration Statement hereby incorporates by reference the contents of the Registrant’s Registration Statement on Form S-8 filed with the Commission on September 27, 2018 (Registration No. 333-227551), to the extent not superseded hereby.

PART II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.Incorporation of Documents by Reference.

The following documents filed by the Registrant with the Commission pursuant to the Securities Act and the Securities Exchange Act of 1934, as amended (the “Exchange Act”) are incorporated herein by reference:

(a) the Registrant’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Commission on March 29, 2019 pursuant to Section 13 of the Exchange Act;

(b) all other reports filed pursuant to Section 13(a) or 15(d) of the Exchange Act since the end of the fiscal year covered by the Registrant’s Annual Report on Form 10-K referred to in (a) above; and

(c) the description of the Registrant’s common stock contained in the Registrant’s registration statement on Form 8-A (File No. 001-38662) filed on September 18, 2018 under Section 12(b) of the Exchange Act, including any amendment or report filed for the purpose of updating such description.

All documents and definitive proxy or information statements filed by the Registrant pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the filing of this Registration Statement and prior to the filing of a post-effective amendment, which indicates that all securities offered hereby have been sold or which deregisters all securities then remaining unsold, shall be deemed to be incorporated by reference into this Registration Statement and to be a part hereof from the date of filing such documents, except as to documents or information deemed to have been furnished and not filed in accordance with the rules of the Commission. Unless expressly incorporated into this Registration Statement, a report deemed to be furnished but not filed on Form 8-K prior or subsequent to the date hereof shall not be incorporated by reference into this Registration Statement. Any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes of this Registration Statement to the extent that a statement contained in any subsequently filed document which also is deemed to be incorporated by reference herein modifies or supersedes such statement.

Item 5. Interests of Named Experts and Counsel.

Not applicable.

Item 8. Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in South San Francisco, California, on this 29th day of March, 2019.

POWER OF ATTORNEY

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints William J. Newell and Edward Albini, and each of them, as his or her true and lawful attorney-in-fact and agent with full power of substitution, for him or her in any and all capacities, to sign any and all amendments (including post-effective amendments) to this Registration Statement on Form S-8, and to file the same, with all exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorney-in-fact and agent full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully for all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorney-in-fact and agent, or his or her substitute, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act, this Registration Statement has been signed by the following persons in the capacities and on the date indicated.