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 ☐

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 under §240.14a-12

89bio, Inc.

(Name of Registrant as Specified in Its Charter)

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

Payment of Filing Fee (Check all boxes that apply):

☒ 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.

*The company will be hosting the meeting live via the Internet this year at the website address above. Please check the proxy materials for instructions on how to access the meeting. Your Vote Counts! 89BIO, INC. 2024 Annual Meeting Vote by May 28, 2024 11:59 PM ET 89BIO, INC. 142 SANSOME STREET, SECOND FLOOR SAN FRANCISCO, CA 94104 You invested in 89BIO, 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 materials for the stockholder meeting to be held on May 29, 2024. Get informed before you vote View the Notice, Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the proxy materials by requesting prior to May 15, 2024. If you would like to request a paper or email copy of the proxy materials or materials for future stockholder 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. There is no charge to you for requesting a copy. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.com Control # Smartphone users Point your camera here and vote without entering a control number Vote Virtually at the Meeting* May 29, 2024 9:00 AM PDT Virtually at: www.virtualshareholdermeeting.com/ETNB2024

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials at www.proxyvote.com or easily request a paper copy. We encourage you to access and review all of the important information contained in the proxy materials before voting. Please follow the instructions on the reverse side to vote on these important matters. Board Voting Items Recommends 1. Election of Directors: To be elected for terms expiring in 2027. Nominees: For 01) Steven Altschuler, M.D. 02) Michael Hayden, M.B. Ch.B., Ph.D. 03) Kathleen LaPorte 2. Ratification of the appointment of KPMG LLP as our independent registered public accounting firm for the year For ending December 31, 2024. 3. Approval of, on an advisory basis, the compensation of the Company’s named executive officers. For 4. Advisory vote on the frequency of future advisory votes on executive compensation. 1 Year NOTE: Such other business as may properly come before the meeting or any adjournment or postponement thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”.