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): November 19, 2013

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.):

Item 1.01 Entry into a Material Definitive Agreement.

Underwriting Agreement

On November 19, 2013, Tesoro Logistics LP (the “Partnership”) and Tesoro Logistics GP, LLC (the “General Partner”) entered into an underwriting agreement (the “Underwriting Agreement”) with Morgan Stanley & Co. LLC, as representative of the several underwriters named therein (collectively, the “Underwriters”), which provides for the issuance and sale by the Partnership, and the purchase by the Underwriters, of an aggregate of 6,300,000 common units representing limited partner interests in the Partnership (the “Common Units”) at a price of $51.05 per Common Unit ($49.26 per Common Unit, net of underwriting discounts and commissions). Pursuant to the Underwriting Agreement, the Partnership granted the Underwriters a 30-day option to purchase up to an additional 945,000 Common Units. 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 November 18, 2013 and the prospectus supplement dated November 19, 2013 filed with the Securities and Exchange Commission pursuant to Rule 424(b) of the Securities Act on November 18, 2013 and November 19, 2013, respectively. The closing of the offering is expected to occur on or about November 22, 2013, subject to customary closing conditions.

The Partnership intends to use the net proceeds from the sale of the Common Units, including the proceeds from any exercise of the Underwriters’ option to purchase additional Common Units, to fund a portion of the cash consideration for the Partnership’s announced acquisition of the majority of the remaining logistics assets (the “Los Angeles Logistics Assets”), that were acquired by a subsidiary of Tesoro Corporation on June 1, 2013. The closing of the acquisition of the Los Angeles Logistics Assets is not conditioned on the closing of the offering, and the offering is not conditioned on the closing of the acquisition of the Los Angeles Logistics Assets. If the acquisition of the Los Angeles Logistics Assets is not consummated for any reason, the Partnership may use the net proceeds from the offering for general partnership purposes, including future acquisitions and capital program expenditures.

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

Certain of 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 Morgan Stanley & Co. LLC, RBC Capital Markets, LLC, Deutsche Bank Securities Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated, UBS Securities LLC, Citigroup Global Markets Inc., J.P. Morgan Securities LLC, Wells Fargo Securities, LLC, Barclays Capital Inc., and Raymond James & Associates, Inc. are lenders under the Partnership’s credit facility.

Item 7.01 Regulation FD Disclosure.

On November 19, 2013, the Partnership issued a press release regarding the pricing of the offering. A copy of the press release is attached hereto as Exhibit 99.1 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, 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: November 20, 2013

Index to Exhibits