UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

SUTRO BIOPHARMA, INC.

(Name of Registrant as Specified In Its Charter)

N/A

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

☒ No fee required.

☐ Fee paid previously with preliminary materials.

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

Your Vote Counts! Get informed before you vote View the Notice of Proxy Statement, Annual Report,1234567890123456789012345678901234567890, 123456789012345678 9012345678901234567890, 1234567890123456789012345678901234567890, 12345678901234567890123456789012345 67890 online OR you can receive a free paper copy of voting material(s) by requesting prior to <matcutoff>. If you would like to request a copy of the voting material(s), you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Smartphone users Point your camera here and vote without entering a control number For complete information and to vote, visit www.ProxyVote.com Control # FLASHID-JOB# Ricky Campana P.O. Box 123456 Suite 500 51 Mercedes Way Edgewood, NY 11717 148,294 322,224 1 OF 2 30# XXXX XXXX XXXX XXXX SUTRO BIOPHARMA, INC. 2023 Annual Meeting Vote by June 07, 2023 11:59 PM ET SUTRO BIOPHARMA, INC. 111 OYSTER POINT BOULEVARD SOUTH SAN FRANCISCO, CALIFORNIA 94080 You invested in SUTRO BIOPHARMA, INC. and it's time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the shareholder meeting to be held on June 08, 2023. View the Notice & Proxy Statement, Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 25, 2023. If you would like to request a copy of the material(s) for this and/or future shareholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. Vote Virtually at the Meeting* June 08, 2023 12:00 PM PDT Virtually at: www.virtualshareholdermeeting.com/STRO2023 *Please check the meeting materials for any special requirements for meeting attendance.

THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming shareholder meeting. Please follow the instructions on the reverse side to vote these important matters. Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”. Voting Items Board Recommends FLASHID-JOB# Control # XXXX XXXX XXXX XXXX THE COMPANY NAME INC. - COMMON ASDFGHJKL 123456789.1234 THE COMPANY NAME INC. - CLASS A 123456789.1234 THE COMPANY NAME INC. - CLASS B 123456789.1234 THE COMPANY NAME INC. - CLASS C 123456789.1234 THE COMPANY NAME INC. - CLASS D 123456789.1234 THE COMPANY NAME INC. - CLASS E 123456789.1234 THE COMPANY NAME INC. - CLASS F 123456789.1234 THE COMPANY NAME INC. - 401 K 123456789.1234 SHARE CLASSES REPRESENTED FOR VOTING This is an overview of the proposals being presented at the upcoming shareholder meeting. Please follow the instructions on the reverse side to vote these important matters. 1. Election of Directors Nominees: 01) William J. Newell 02) Connie Matsui 03) James Panek For 2. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2023. For 3. To approve, on a non-binding, advisory basis, the compensation of our named executive officers. For 4. To approve the amendment and restatement of our restated certificate of incorporation to permit the exculpation of our officers from personal liability for certain breaches of the duty of care. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof.