DEFA14A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(AMENDMENT NO. )

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

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☐ 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)

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☒ 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.

SUTR·J BIOPHARMA 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 05, 2026.Get informed before you vote 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 22, 2026. 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. Smartphone users Point your camera here and vote without entering a I number Virtually at: Vote Virtually at the Meeting*June 05, 2026 8:00 AM PDT www.virtualshareholdermeeting.com/STR02026*Please check the meeting materials for any special requirements for meeting attendance.

Vote at www.ProxyVote.com 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.Voting Items 1.Election of Class II Directors Nominees:01)Jane Chung, RPh 02)Connie Matsui 03)James Panek Board Recommends 2.To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026.3.To approve, on a non-binding, advisory basis, the compensation of our named executive officers.NOTE: Such other business as may properly come before the meeting or any adjournment thereof.0For OFor Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click "Delivery Settings".

SEC Form DEFA14A filed by Sutro Biopharma Inc.