August 8, 2019

Pablo Cagnoni
Chief Executive Officer
Rubius Therapeutics, Inc.
399 Binney Street, Suite 300
Cambridge, MA 02139

       Re: Rubius Therapeutics, Inc.
           Registration Statement on Form S-3
           Filed August 1, 2019
           File No. 333-232955

Dear Mr. Cagnoni:

       We have limited our review of your registration statement to those
issues we have
addressed in our comments. In some of our comments, we may ask you to provide
us with
information so we may better understand your disclosure.

       Please respond to this letter by amending your registration statement
and providing the
requested information. If you do not believe our comments apply to your facts
and
circumstances or do not believe an amendment is appropriate, please tell us why
in your
response.

       After reviewing any amendment to your registration statement and the
information you
provide in response to these comments, we may have additional comments.

Registration Statement on Form S-3

Description of Capital Stock
Choice of Forum , page 11

1.     Your exclusive forum provision in Section 8 of your bylaws, filed as
Exhibit 3.2, does not
       explicitly address actions brought under the Exchange Act, and your
description of the
       exclusive forum provision on page 11 of your prospectus is inconsistent
with your
       disclosure on page 126 of your Form 10-K for the fiscal year ended
December 31,
       2018 and with your disclosure on page 79 of your registration statement
on Form S-1
       (333-225840). Your prospectus states that the Delaware Forum Provision
does not apply
       to Exchange Act claims and that the Federal Forum Provision applies only
to Securities
       Act claims. Your Form 10-K states that, if the Chancery Court of the
State of Delaware
       does not have jurisdiction, the federal district court for the District
of Delaware is
 Pablo Cagnoni
Rubius Therapeutics, Inc.
August 8, 2019
Page 2
         the exclusive forum for any such claims and that the federal district
court for the District
         of Massachusetts is the exclusive forum for Securities Act claims.
Your registration
         statement on Form S-1 states that the federal district court for the
District of
         Massachusetts is the exclusive forum for Securities Act and Exchange
Act claims. Please
         tell us how Section 8 of your bylaws applies to Exchange Act claims
and amend the
         disclosure on page 11 of your prospectus, if necessary. In this
regard, we note that
         Section 27 of the Exchange Act creates exclusive federal jurisdiction
over all suits brought
         to enforce any duty or liability created by the Exchange Act or the
rules and regulations
         thereunder.
        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.

       Refer to Rules 460 and 461 regarding requests for acceleration. Please
allow adequate
time for us to review any amendment prior to the requested effective date of
the registration
statement.

       Please contact Sonia Bednarowski at 202-551-3666 or Dietrich King at
202-551-8071
with any questions.



FirstName LastNamePablo Cagnoni                                Sincerely,
Comapany NameRubius Therapeutics, Inc.
                                                               Division of
Corporation Finance
August 8, 2019 Page 2                                          Office of
Healthcare & Insurance
FirstName LastName