United States securities and exchange commission logo November 15, 2021 Anna Protopapas President and Chief Executive Officer Mersana Therapeutics, Inc. 840 Memorial Drive Cambridge, Massachusetts 02139 Re: Mersana Therapeutics, Inc. Registration Statement on Form S-3 Filed November 9, 2021 File No. 333-260895 Dear Ms. Protopapas: This is to advise you that we have not reviewed and will not review your registration statement. Please refer to Rules 460 and 461 regarding requests for acceleration. We remind you that the company and its management are responsible for the accuracy and adequacy of their disclosures, notwithstanding any review, comments, action or absence of action by the staff. Please contact Chris Edwards at 202-551-6761 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences cc: William J. Michener, Esq.