Exhibit 5.1

      March 1, 2022                                                              200 Clarendon Street Boston, Massachusetts 02116 Tel: +1.617.948.6000 Fax: +1.617.948.6001 www.lw.com   FIRM / AFFILIATE OFFICES Beijing     Moscow Boston      Munich Brussels      New York Century City    Orange County Chicago      Paris Dubai      Riyadh Düsseldorf     San Diego Frankfurt     San Francisco Hamburg     Seoul Hong Kong     Shanghai Houston      Silicon Valley London       Singapore Los Angeles     Tokyo Madrid     Washington, D.C. Milan           

Seres Therapeutics, Inc.

200 Sidney Street – 4^th Floor

Cambridge, MA 02139

Re: Registration Statement on Form S-8; 4,075,576 shares of Seres Therapeutics, Inc. Common Stock, $0.001 par value per share

To the addressee set forth above:

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 an aggregate of 4,075,576 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.

March 1, 2022

Page 2

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.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Sincerely,

/s/ Latham & Watkins LLP