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): October 22, 2018

PROTEOSTASIS THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

80 Guest Street, Suite 500

Boston, MA 02135

(Address of principal executive offices, including zip code)

(617) 225-0096

(Registrant’s telephone number, including area code)

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.  ☒

Item 1.01 Entry Into a Material Definitive Agreement.

On October 23, 2018, Proteostasis Therapeutics, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Leerink Partners LLC and Piper Jaffray & Co. as representatives of the several underwriters named therein (the “Underwriters”), relating to the underwritten public offering of 11,000,000 shares of the Company’s common stock, par value $0.001 per share (the “Offering”). The price to the public in the Offering was $6.75 per share. The net proceeds to the Company from the Offering are expected to be approximately $74.3 million, after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company. The Offering is scheduled to close on or about October 26, 2018, subject to customary closing conditions. In addition, under the terms of the Underwriting Agreement, the Company granted the Underwriters an option, exercisable for 30 days, to purchase up to 1,650,000 additional shares of common stock.

The Underwriting Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriters, including for liabilities under the Securities Act of 1933, as amended (the “Securities Act”), other obligations of the parties and termination provisions.

The Offering was made pursuant to the Company’s effective registration statement on Form S-3 (Registration Statement No. 333-224180) previously filed with the Securities and Exchange Commission (“SEC”) and a related prospectus supplement and accompanying prospectus. The Underwriting Agreement is filed as Exhibit 1.1 to this report and is incorporated herein by reference. The foregoing description of the terms of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit. A copy of the opinion of Goodwin Procter LLP relating to the legality of the issuance and sale of the shares in the Offering is attached as Exhibit 5.1 hereto.

Item 8.01 Other Events.

On October 22, 2018, the Company issued a press release announcing that it had commenced the Offering. On October 23, 2018, the Company issued a press release announcing that it had priced the Offering.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

INDEX TO 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.