United States securities and exchange commission logo May 13, 2020 Brian DeSchuytner Senior Vice President of Finance and Product Strategy Mersana Therapeutics, Inc. 840 Memorial Drive Cambridge, Massachusetts 02139 Re: Mersana Therapeutics, Inc. Registration Statement on Form S-3 Filed May 8, 2020 File No. 333-238140 Dear Mr. DeSchuytner: 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 Courtney Lindsay at (202) 551-7237 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences