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): September 17, 2014

ARATANA THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

1901 Olathe Blvd., Kansas City, KS 66103

(Address of principal executive offices) (Zip Code)

(913) 353-1000

(Registrant’s telephone number, include 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 (see General Instruction A.2. below):

On September 17, 2014, Aratana Therapeutics, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Jefferies LLC and Barclays Capital Inc., as underwriters (the “Underwriters”), in connection with the public offering, issuance and sale by the Company of 4,500,000 shares of the Company’s common stock, par value $0.001 per share, at the public offering price of $9.25 per share, less underwriting discounts and commissions, pursuant to an effective registration statement on Form S-3 (File No. 333-197414) and a related prospectus supplement filed with the Securities and Exchange Commission. Under the terms of the Underwriting Agreement, the Company also granted the Underwriters an option exercisable for 30 days to purchase up to an additional 675,000 shares of its common stock at the public offering price, less underwriting discounts and commissions. On September 18, 2014, the Underwriters exercised their option to purchase additional shares in full.

The Company expects to receive net proceeds from the offering of approximately $44.8 million, after deducting underwriting discounts and commissions and estimated offering expenses. The Company intends to use the net proceeds of the offering for the development of its therapeutic candidates, expansion of its commercial infrastructure in anticipation of future product launches and for other general corporate and working capital purposes.

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, other obligations of the parties and termination provisions.

The above description of the Underwriting Agreement is qualified in its entirety by reference to the full text of the Underwriting Agreement, a copy of which is filed as Exhibit 1.1 hereto and is incorporated herein by reference.

Latham & Watkins LLP, counsel to the Company, has issued an opinion to the Company, dated September 19, 2014, regarding the validity of the shares of common stock to be issued and sold in the offering. A copy of the opinion is filed as Exhibit 5.1 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.

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