X0508 4 2023-10-27 0 0001609809 Seres Therapeutics, Inc. MCRB 0001435744 DesRosier Thomas C/O SERES THERAPEUTICS, INC. 200 SIDNEY STREET CAMBRIDGE MA 02139 0 1 0 0 Chief Legal Officer and EVP 1 Common Stock 2023-10-27 4 M 0 13501 0.00 A 118047 D Common Stock 2023-10-30 4 S 0 4156 1.35 D 113891 D Restricted Stock Units 2023-04-27 4 A 0 27001 0.00 A Common Stock 27001 27001 D Restricted Stock Units 2023-10-27 4 M 0 13501 0.00 D Common Stock 13501 13500 D On February 3, 2023, the reporting person was granted an award of 27,001 restricted stock units ("RSUs"), which vest in the form of shares of the Issuer's common stock in two substantially equal installments upon the satisfaction of certain performance criteria. On April 27, 2023 (the "Initial Milestone Date"), the Issuer's Administrator determined that the performance criteria for the first installment were met, resulting in the vesting and settlement of the award as to 13,501 RSUs on October 27, 2023, the date that was six months from the Initial Milestone Date. The remaining 13,500 RSUs shall vest on October 27, 2024, the date that is eighteen months from the Initial Milestone Date. The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 instruction adopted by the reporting person on May 4, 2023, solely with the intent to cover taxes in connection with the vesting of the RSUs. The price reported in column 4 is a weighted average of the price. These shares were sold in multiple transactions at prices ranging from $1.35 to $1.385, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. Each RSU represents a contingent right to receive one share of Issuer common stock. The RSUs do not have an expiration date. /s/ Thomas J. DesRosier 2023-10-31