UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): June 29, 2020

RUBIUS THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

(617) 679-9600

(Registrant’s telephone number, including area code)

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 or Rule 12b-2 of the Securities Exchange Act of 1934.

Emerging growth company x

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ¨

Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On June 30, 2020, Rubius Therapeutics, Inc. (the “Company”) announced that Andrew Oh will step down as the Chief Financial Officer, effective as of December 31, 2020 (the “Anticipated Separation Date”), or such sooner time as determined by the Company. In connection with his departure, the Company and Mr. Oh entered into a transitional services and separation agreement dated as of June 29, 2020 (the “Separation Agreement”).

Consistent with the terms of the Employment Agreement dated June 29, 2018 between the Company and Mr. Oh, the Separation Agreement provides that Mr. Oh will be entitled to receive, among other things, the following: (i) salary continuation for nine (9) months following the Anticipated Separation Date and (ii) up to nine (9) months of continued COBRA coverage for Mr. Oh and his eligible dependents at the Company’s normal rate of contribution for employees in effect immediately prior to the Separation Date. The Separation Agreement contains confidentiality, non-disparagement and non-solicitation covenants and a release of claims by Mr. Oh. The foregoing is only a summary description of the terms of the Separation Agreement, does not purport to be complete and is qualified in its entirety by reference to the Separation Agreement, which will be filed as an exhibit to the Company’s Quarterly Report on Form 10-Q for the fiscal quarter ending June 30, 2020.

Item 8.01 Other Events.

On June 30, 2020, the Company issued a press release announcing the transition of Mr. Oh and providing an update on clinical trial progress for its lead oncology product candidate, RTX-240. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.