Material Event (Form 8-K)

Seres Therapeutics, Inc. (the "Company") announced today that the Company's 2026 annual meeting of stockholders (the "2026 Annual Meeting") has been scheduled for June 9, 2026. All holders of record of common stock outstanding as of the close of business on April 13, 2026 will be entitled to vote at the 2026 Annual Meeting. The time and location for the 2026 Annual Meeting will be set forth in the Company's definitive proxy statement for the 2026 Annual Meeting.

As the 2026 Annual Meeting is being held more than 30 days after the anniversary of the Company's 2025 annual meeting of stockholders under Rule 14a-5(f)under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), the Company is hereby providing notice of the below revised deadline for qualified stockholder proposals bought pursuant to Rule 14a-8of Exchange Act.

In order for a stockholder proposal for the 2026 Annual Meeting to be eligible for inclusion in the Company's proxy statement pursuant to Rule 14a-8of the Exchange Act, the Company must receive the proposal and supporting statements at its principal executive office no later than the close of business on March 26, 2026 (which is the tenth (10th) day following the date of this Current Report on Form 8-K).