X0306 4 2018-06-20 0 0001609809 Seres Therapeutics, Inc. MCRB 0001614936 Pomerantz Roger C/O SERES THERAPEUTICS, INC. 200 SIDNEY STREET CAMBRIDGE MA 02139 1 1 0 0 President and CEO Common Stock 2018-06-20 4 S 0 26492 9.3255 D 297812 D Common Stock 2018-06-21 4 S 0 18987 9.0638 D 278825 D Common Stock 2018-06-22 4 S 0 40057 9.3557 D 238768 D The sale reported in the Form 4 was effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on March 15, 2018. The price reported is a weighted average price. The shares were sold in multiple transactions at per share prices ranging from $9.00 to $9.60. The Reporting Person undertakes to provide upon request to the SEC staff, the Issuer, or any stockholder of the Issuer, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The price reported is a weighted average price. The shares were sold in multiple transactions at per share prices ranging from $9.00 to $9.55. The Reporting Person undertakes to provide upon request to the SEC staff, the Issuer, or any stockholder of the Issuer, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The price reported is a weighted average price. The shares were sold in multiple transactions at per share prices ranging from $9.00 to $9.74. The Reporting Person undertakes to provide upon request to the SEC staff, the Issuer, or any stockholder of the Issuer, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. /s/ Thomas J. DesRosier, Attorney-in-Fact 2018-06-22