Seres Therapeutics Inc

This proxy statement is furnished in connection with the solicitation by the Board of Directors of Seres Therapeutics, Inc. of proxies to be voted at our Annual Meeting of Stockholders to be held on Thursday, April 4, 2024 (the “Annual Meeting”), at 8:00 a.m. Eastern time, and at any continuation, postponement, or adjournment of the Annual Meeting. The Annual Meeting will be a completely virtual meeting, which will be conducted via live webcast. You will be able to attend the Annual Meeting online and submit your questions during the meeting by visiting www.virtualshareholdermeeting.com/ MCRB2024 and entering the 16-digit control number included on your proxy card or on the instructions that accompanied your proxy materials. Holders of record of shares of our common stock (our “Common Stock”), as of the close of business on February 12, 2024 (the “Record Date”), will be entitled to notice of and to vote at the Annual Meeting and any continuation, postponement, or adjournment of the Annual Meeting. As of the Record Date, there were 145,557,567

shares of Common Stock outstanding and entitled to vote at the Annual Meeting. Each share of Common Stock is entitled to one vote on any matter presented to stockholders at the Annual Meeting.

This proxy statement and our Annual Report to Stockholders for the year ended December 31, 2023 (the “2023 Annual Report”) will be released on or about March 5, 2024 to our stockholders on the Record Date.

In this proxy statement, “Seres”, “Company”, “we”, “us”, and “our” refer to Seres Therapeutics, Inc.

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE STOCKHOLDER MEETING TO BE HELD ON THURSDAY, APRIL 4, 2024

This Proxy Statement and our 2023 Annual Report to Stockholders are available at http://www.proxyvote.com/.