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 20, 2018

MAGENTA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

50 Hampshire Street

Cambridge, MA 02139

(Address of principal executive offices, including zip code)

(857) 242-0170

(Registrant’s telephone number, including area code)

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 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

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.  ☐

As previously disclosed in the Registration Statement on Form S-1 (File No. 333-225178) (the “Registration Statement”) of Magenta Therapeutics, Inc. (the “Company”), on May 24, 2018 and in connection with the consummation of the Company’s initial public offering, the Company filed an amended and restated certificate of incorporation (the “Restated Certificate”) with the Secretary of State of the State of Delaware. The Board of Directors (the “Board”) and the Company’s stockholders previously approved the Restated Certificate to be filed in connection with, and to be effective upon, the consummation of the initial public offering. The Restated Certificate amends and restates the Company’s existing third amended and restated certificate of incorporation in its entirety to, among other things: (i) authorize 150,000,000 shares of common stock (the “Common Stock”); (ii) eliminate all references to the previously-existing series of preferred stock; and (iii) authorize 10,000,000 shares of undesignated preferred stock that may be issued from time to time by the Board in one or more series.

The foregoing description of the Restated Certificate is qualified by reference to the Restated Certificate, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.

In addition, as previously disclosed in the Registration Statement, on May 24, 2018 and in connection with the consummation of the Company’s initial public offering, the amended and restated by-laws of the Company (the “Amended and Restated By-Laws”), previously approved by the Board and the Company’s stockholders to become effective immediately upon the effectiveness of the Registration Statement, became effective on June 20, 2018. The Amended and Restated By-Laws amend and restate the Company’s by-laws in their entirety to, among other things: (i) eliminate the ability of the Company’s stockholders to take action by written consent in lieu of a meeting and call special meetings of stockholders; (ii) establish procedures relating to the presentation of stockholder proposals at stockholder meetings; (iii) establish procedures relating to the nomination of directors; and (iv) conform to the amended provisions of the Restated Certificate.

The foregoing description of the Amended and Restated By-Laws is qualified by reference to the Amended and Restated By-Laws, a copy of which is attached hereto as Exhibit 3.2 and is incorporated herein by reference.

On June 25, 2018, the Company completed its initial public offering of 6,666,667 shares of Common Stock at a price to the public of $15.00 per share for gross proceeds of approximately $100.0 million.

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.