UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A INFORMATION

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Securities Exchange Act of 1934

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SERES THERAPEUTICS, INC.

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Your Vote Counts! SERES THERAPEUTICS, INC. 2022 Annual Meeting Vote by June 21, 2022 11:59 PM ET SERES THERAPEUTICS, INC. 200 SIDNEY STREET CAMBRIDGE, MA 02139 D82370-P68775 You invested in SERES THERAPEUTICS, 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 stockholder meeting to be held on June 22, 2022. Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 8, 2022. If you would like to request a copy of the material(s) for this and/or 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. 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 Vote Virtually at the Meeting* Point your camera here and June 22, 2022 8:00 AM EDT vote without entering a control number Virtually at: www.virtualshareholdermeeting.com/MCRB2022 *Please check the meeting materials for any special requirements for meeting attendance. V1.1 SERES

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Voting Items Board Recommends 1. Election of Directors Nominees: 01) Dennis A. Ausiello, M.D. For 02) Willard H. Dere, M.D. 03) Eric D. Shaff 2. Ratify the appointment of PricewaterhouseCoopers LLP as our independent registered public accounting firm for the For fiscal year ending December 31, 2022. 3. Approval, on an advisory (non-binding) basis, of the compensation of our named executive officers. For NOTE: To transact such other business as may properly come before the Annual Meeting or any continuation, postponement, or adjournment thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”. D82371-P68775