March 6, 2019
200 Sidney Street
Cambridge, MA 02139
Registration Statement on Form S-8; 2,037,469 shares of Seres
Common Stock, $0.001 par value per share
Ladies and Gentlemen:
We have acted as special
counsel to Seres Therapeutics, Inc., a Delaware corporation (the Company), in connection with the preparation and filing by the Company with the Securities and Exchange Commission (the Commission)
of a registration statement on Form S-8 (the Registration Statement) under the Securities Act of 1933, as amended (the Act), relating to the issuance by the
Company of up to 2,037,469 shares of common stock of the Company, $0.001 par value per share (the Shares), issuable under the Seres Therapeutics, Inc. 2015 Incentive Award Plan (the 2015 Plan) and
the Seres Therapeutics, Inc. 2015 Employee Stock Purchase Plan (together with the 2015 Plan, the Plans). This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or the prospectus forming a part thereof, other than as
expressly stated herein with
respect to the issuance of the Shares.
As such counsel, we have examined such matters of fact and questions of law as we have considered
appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining
herein as to the General Corporation Law of the State of Delaware (the DGCL), and we express no opinion with respect to any other laws.
Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, when the Shares shall have
been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, and have been issued by the Company for legal consideration in excess of par value in the circumstances contemplated by the
Plans, assuming in each case that the individual grants or awards under the Plans are duly authorized by all necessary corporate action and duly granted or awarded and exercised in accordance with the requirements of law and the Plans (and the
agreements and awards duly adopted thereunder and in accordance therewith), the issue and sale of the Shares will have been duly authorized by all necessary corporate action of the Company, and the Shares will be validly issued, fully paid and
nonassessable. In rendering the foregoing opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.