Seres Therapeutics, Inc.

a)

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

Item 5.02. Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

As previously reported, on October 22, 2026, Marella Thorell resigned from her positions as Executive Vice President and Chief Financial Officer of the Company, effective November 15, 2026 (the “Effective Date”).

On November 1, 2026, the Board appointed Richard Kender, the Company’s Executive Chair and Interim Chief Executive Officer and principal executive officer, to serve as the Company’s principal financial officer and principal accounting officer and succeed Ms. Thorell in these capacities, effective as of the Effective Date. Mr. Kender will continue to serve as the Company’s Executive Chair and Interim Chief Executive Officer and principal executive officer, and he will not receive any additional compensation for his role as principal financial officer and principal accounting officer during this period.

A description of Mr. Kender’s business experience can be found under the section entitled "Executive Officers" in the Company’s definitive proxy statement on Schedule 14A, filed with the Securities and Exchange Commission on April 27, 2026, and which biography is incorporated by reference herein.

The Company and Mr. Kender have previously entered into the Company’s standard indemnification agreement for executive officers. There are no family relationships between Mr. Kender and any director or executive officer of the Company that would be required to be disclosed pursuant to Item 401(d) of Regulation S-K, and Mr. Kender does not have a material interest in any transaction that is required to be disclosed under Item 404(a) of Regulation S-K.

Advisory Agreement with Marella Thorell

In connection with her resignation, on November 2, 2026, the Company and Ms. Thorell entered into an advisory agreement (the "Advisory Agreement") pursuant to which Ms. Thorell will provide advisory services to the Company from the Effective Date through March 31, 2027, unless terminated earlier by either party on 15 business days' prior written notice, in exchange for an hourly fee of $500.00. Ms. Thorell is expected to provide the Company with up to 20 hours of advisory services per month. Ms. Thorell's performance of advisory services will constitute continued service to the Company for purposes of the vesting and exercisability of her outstanding Company stock options.

Resignation of Robert Rosiello

On October 30, 2026, Robert Rosiello resigned from his position as a member of the board of directors of the Company and on the committees on which he serves, effective October 30, 2026. Mr. Rosiello's departure is not due to any disagreement with the Company on any matter relating to the Company’s operations, policies or practices.

b)

Material changes to the procedures by which security holders may recommend nominees to the board of directors.

None.

c)

Insider trading arrangements and policies.

During the three months ended September 30, 2026, no director or "officer" (as defined in Rule 16a-1(f) under the Exchange Act) of the Company adopted orterminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408(a) of Regulation S-K.