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): January 3, 2019

UNUM THERAPEUTICS INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code (617) 945-5576

Not Applicable

(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:

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  ☒

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.  ☒

On January 3, 2019, Unum Therapeutics Inc. (the “Company”) announced Christiana Stamoulis will resign as Chief Financial Officer (“CFO”) and President of the Company effective as of January 31, 2019. Ms. Stamoulis’ resignation did not result from a disagreement with the Company on any matter relating to the Company’s operations, policies or practices, including its controls or financial related matters. The Company has initiated a search to identify a replacement CFO.

On January 3, 2019, the Board of Directors of the Company (the “Board”) appointed Charles Wilson, Ph.D. as President of the Company, effective as of the date of Ms. Stamoulis’ resignation. Mr. Wilson currently serves as the Chief Executive Officer of the Company and will continue in that role while serving as President. The Board also appointed John Green, Senior Director of Finance and Controller, as the “principal financial officer” and “principal accounting officer” of the Company, effective as of the date of Ms. Stamoulis’ resignation.

On January 3, 2019, the Company issued a press release announcing the resignation of Christiana Stamoulis as CFO and President of the Company and the appointment of Charles Wilson as President of the Company. A copy of the press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K.

In addition, on January 3, 2019, the Company issued a press release announcing 2019 goals and anticipated milestones for 2019. A copy of the press release is furnished as Exhibit 99.2 to this Current Report on Form 8-K.

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.

Exhibit 99.1

Unum Therapeutics Announces Transition of Chief Financial Officer

CAMBRIDGE, MA, January 3, 2018 – Unum Therapeutics Inc. (NASDAQ: UMRX), a clinical-stage biopharmaceutical company focused on the development of novel cellular immunotherapies, today announced that Christiana Stamoulis, President and Chief Financial Officer, will be stepping down from the Company, effective January 31, 2019, to pursue another opportunity. Ms. Stamoulis will work closely with management to ensure a smooth transition to her successor.

“I want to thank Christiana for her tireless efforts over the last four years as we have worked to build a leading publicly-traded, clinical-stage cell therapy company with a broad and growing pipeline,” said Dr. Wilson. “Christiana has played a pivotal role in supporting our growth, and we wish her the best as she explores the next chapter in her career.”

“I am fortunate to have worked at such a remarkable organization and to have been a part of Unum’s transformation from a very early-stage startup into a fast-growing public company in the high-potential field of cancer cell therapy,” said Ms. Stamoulis. “With its exceptional team, promising emerging data, and rich clinical pipeline of products, Unum is well-positioned for the future, and I look forward to watching its success in the years ahead.”

Unum is initiating a search for its next Chief Financial Officer, who will play an important role in continuing to support the next stage of the Company’s growth. Following Ms. Stamoulis’ departure, certain members of management and the Company’s existing finance team will assume the duties and responsibilities of the CFO office on an interim basis. Dr. Chuck Wilson, CEO, Unum Therapeutics, will reassume the role of President.

Forward looking Statements

This press release contains forward-looking statements. Statements in this press release about our future expectations, plans and prospects, including statements regarding the changes in our officer positions, the Company’s plan to appoint a new Chief Financial Officer, the anticipated timing of our clinical trials and regulatory filings, our long-term growth and our ability to achieve our strategy, the development of our product candidates, including the three lead ACTR product candidates, the results of our clinical trials, as well as other statements containing the words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “might,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “will,” or “would” and similar expressions, constitute forward-looking statements within the meaning of the safe harbor provisions of The Private Securities Litigation Reform Act of 1995. We may not actually achieve the forecasts disclosed in our forward-looking statements, and you should not place undue reliance on our forward-looking statements. Actual results could differ materially from the projections disclosed in the

forward-looking statements we make as a result of a variety of risks and uncertainties, including risks related to the accuracy of our estimates regarding expenses, future revenues, capital requirements, and the need for additional financing, the success, cost and timing of our product development activities and clinical trials, our ability to obtain and maintain regulatory approval for our product candidates, and the other risks and uncertainties described in the “Risk Factors” sections of our public filings with the Securities and Exchange Commission. In addition, the forward-looking statements included in this press release represent our views as of the date hereof. We anticipate that subsequent events and developments may cause our views to change. However, while we may elect to update these forward-looking statements at some point in the future, we specifically disclaim any obligation to do so. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date hereof.

Investor Contact:

Stephanie Ascher, 212-362-1200

stephanie@sternir.com

Media Contact:

Paul Kidwell, 617-680-1088

paul.kidwell@unumrx.com

Exhibit 99.2

Unum Therapeutics Announces 2019 Goals and Expected Milestones

-Multiple Data Readouts from ATTCK-20-03 and ATTCK-17-01 Expected in 2019 -

- Initial Data on ATTCK-34-01, Unum’s First Program in Solid Tumors, Expected in Second Half of 2019 -

- First Development Candidate from BOXR Platform Advancing Toward Clinical Development -

CAMBRIDGE, MA, January 3, 2018 – Unum Therapeutics Inc. (NASDAQ: UMRX), a clinical-stage biopharmaceutical company focused on the development of novel cellular immunotherapies, today announced its anticipated milestones for 2019.

“Unum made substantial progress in 2018 as we reported early data from our programs in non-Hodgkin lymphoma and multiple myeloma, while simultaneously expanding applications of our ACTR platform into solid tumors and introducing a second novel technology platform, BOXR, designed to improve the functionality of engineered T cells,” said Chuck Wilson, CEO of Unum. “We expect 2019 also to be a year of significant momentum, with data expected from all four of our ongoing clinical programs, including readouts from the ATTCK-20-03 and ATTCK-17-01 trials, as well as an initial data readout from our first study in solid tumors. Additionally, we continue to innovate in the field of cell therapy and have recently nominated our first development candidate from our BOXR technology platform to advance toward clinical development.”

Anticipated 2019 Milestones

ACTR707 + rituximab in r/r NHL

ACTR087 + SEA-BCMA in r/r multiple myeloma

ACTR707 + trastuzumab in HER2+ advanced cancers

ACTR087 + rituximab in r/r NHL

BOXR Platform

About Unum Therapeutics

Unum Therapeutics is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel immunotherapy products designed to harness the power of a patient’s immune system to cure cancer. Unum’s novel proprietary technologies include Antibody-Coupled T cell Receptor (ACTR), a universal, engineered cell therapy intended to be used in combination with a wide range of tumor-specific antibodies to target different tumor types, and Bolt-On Chimeric Receptor (BOXR), an approach for improving T cell functionality to enable solid tumor cancer applications. Unum has four product candidates currently in Phase I clinical testing, including: ACTR707 used in combination with rituximab, an anti-CD20 antibody, in adult patients with relapsed or refractory non-Hodgkin lymphoma (r/r NHL); ACTR087 used in combination with the novel antibody SEA-BCMA in adult patients with relapsed or refractory multiple myeloma; and ACTR707 used in combination with trastuzumab, an anti-human epidermal growth factor receptor 2 (HER2) antibody, in adult patients with HER2+ advanced cancer.

The Company is headquartered in Cambridge, MA.

Forward looking Statements

This press release contains forward-looking statements. Statements in this press release about our future expectations, plans and prospects, including projections regarding future revenues and financial performance, the anticipated timing of our clinical trials and regulatory filings, our long-term growth and our ability to achieve our strategy, the design of our clinical trials, the development of our product candidates, including the three lead ACTR product candidates, the results of our clinical trials, as well as other statements containing the words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “might,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “will,” or “would” and similar expressions, constitute forward-looking statements within the meaning of the safe harbor provisions of The Private Securities Litigation Reform Act of 1995. We may not actually achieve the forecasts disclosed in our forward-looking statements, and you should not place undue reliance on our forward-looking statements. Actual results could differ materially from the projections disclosed in the forward-looking statements we make as a result of a variety of risks and uncertainties, including risks related to the accuracy of our estimates regarding expenses, future revenues, capital requirements, and the need for additional financing, the success, cost and timing of our product development activities and clinical trials, our ability to obtain and maintain regulatory approval for our product candidates, and the other risks and uncertainties described in the “Risk Factors” sections of our public filings with the Securities and Exchange Commission. In addition, the forward-looking statements included in this press release represent our views as of the date hereof. We anticipate that subsequent events and developments may cause our views to change. However, while we may elect to update these forward-looking statements at some point in the future, we specifically disclaim any obligation to do so. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date hereof.

Investor Contact:

Stephanie Ascher, 212-362-1200

stephanie@sternir.com

Media Contact:

Paul Kidwell, 617-680-1088

paul.kidwell@unumrx.com