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): August 18, 2014

TESORO LOGISTICS LP

(Exact name of registrant as specified in its charter)

(210) 626-6000

(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 (see General Instruction A.2.):

Underwriting Agreement

On August 18, 2014, Tesoro Logistics LP (the “Partnership”) and Tesoro Logistics GP, LLC (the “General Partner”) entered into an underwriting agreement (the “Underwriting Agreement”) with UBS Securities LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated (collectively, the “Underwriters”), which provides for the issuance and sale by the Partnership, and the purchase by the Underwriters, of an aggregate of 2,100,000 common units representing limited partner interests in the Partnership (the “Common Units”) at a price of $67.47 per Common Unit. The offer and sale of the Common Units have been registered under the Securities Act of 1933, as amended (the “Securities Act”), pursuant to an effective Registration Statement on Form S-3 (Registration No. 333-185926) of the Partnership, as supplemented by the preliminary prospectus supplement dated August 18, 2014 and the prospectus supplement dated August 18, 2014 filed with the Securities and Exchange Commission pursuant to Rule 424(b) of the Securities Act on August 18, 2014 and August 20, 2014, respectively. The closing of the offering is expected to occur on or about August 22, 2014, subject to customary closing conditions.

The net proceeds of the offering after deducting underwriting discounts and commissions and estimated expenses were approximately $141.5 million. The Partnership intends to use the net proceeds from the sale of the Common Units to redeem a portion of its 5.875% Senior Notes due 2020.

The Underwriting Agreement contains customary representations, warranties and agreements of the Partnership and the General Partner, including obligations of the Partnership and the General Partner to indemnify the Underwriters for certain liabilities under the Securities Act and to contribute to payments the Underwriters may be required to make because of any of those liabilities. The foregoing description of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Underwriting Agreement, which is filed as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Relationships

The Underwriters and their related entities have engaged, and may in the future engage, in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. The Underwriters and their related entities have performed and may perform investment and commercial banking and advisory services for the Partnership and its affiliates from time to time, for which they have received and may receive customary fees and expense reimbursement. The Underwriters and their affiliates may, from time to time, engage in transactions with and perform services for the Partnership in the ordinary course of their business. Affiliates of UBS Securities LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated are lenders under the Partnership’s credit facility.

On August 18, 2014, the Partnership issued a press release announcing the commencement of a registered underwritten public offering of 2,100,000 common units representing limited partner interests in the Partnership. The press release related to this announcement is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

On August 19, 2014, the Partnership issued a press release regarding the pricing of the offering. A copy of the press release is attached hereto as Exhibit 99.2 and is incorporated herein by reference.

The information above under Item 7.01 is being furnished, not filed, pursuant to Item 7.01 of Form 8-K. Accordingly, the information in Item 7.01 of this Current Report, including Exhibit 99.1 and Exhibit 99.2, will not be subject to liability under Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and will not be incorporated by reference into any registration statement or other document filed by the Partnership under the Securities Act or the Exchange Act, unless specifically identified therein as being incorporated 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.

Date: August 20, 2014

Index to Exhibits