Seres Therapeutics, Inc.

a)

Disclosure in lieu of reporting on a Current Report on Form 8-K.

None.

b)

Insider Trading Arrangements and Policies.

On November 25, 2025, Teresa L. Young, Ph.D., our Chief Commercial and Strategy Officer, adopted a Rule 10b5-1 plan (the “Young 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 31,680 sharesof common stock and (2) the net number of shares of common stock underlying 1,152 unvested restricted stock units after giving effect to the number of shares to be sold outside of the Young 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and March 12, 2027, unless otherwise terminated according to its terms.

On November 26, 2025, Kelly Brady, M.S., our Executive Vice President, Chief Operating Officer, adopted a Rule 10b5-1 plan (the “Brady 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 7,611 shares of common stock and (2) the net number of shares of common stock underlying 797 unvested restricted stock units after giving effect to the number of shares to be sold outside of the Brady 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and November 17, 2026, unless otherwise terminated according to its terms.

Other than the foregoing, no director or officer of the Company adopted or terminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408 of Regulation S-K.