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

August 22, 2013

Date of Report (Date of earliest event reported)

TARGA RESOURCES PARTNERS LP

(Exact name of Registrant as specified in its charter)

1000 Louisiana Street, Suite 4300

Houston, TX 77002

(Address of principal executive offices)

(713) 584-1000

(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:

Item 1.01. Entry into a Material Definitive Agreement.

On August 22, 2013, Targa Resources Partners LP (the “Partnership”) entered into an Equity Distribution Agreement (the “Agreement”) with Citigroup Global Markets Inc., Deutsche Bank Securities Inc., Morgan Stanley & Co. LLC, Raymond James & Associates, Inc., RBC Capital Markets, LLC, UBS Securities LLC and Wells Fargo Securities, LLC (each, a “Manager” and collectively, the “Managers”). Pursuant to the terms of the Agreement, the Partnership may sell from time to time through the Managers, as the Partnership’s sales agents, the Partnership’s common units representing limited partner interests having an aggregate offering price of up to $400,000,000 (the “Units”). Sales of the Units, if any, will be made by means of ordinary brokers’ transactions on the New York Stock Exchange, any other national securities exchange or facility thereof, a trading facility of a national securities association or an alternate trading system, to or through a market maker or directly on or through an electronic communication network, a “dark pool” or any similar market venue, at market prices, in block transactions or as otherwise agreed by the Partnership and one or more of the Managers.

Under the terms of the Agreement, the Partnership may also sell Units from time to time to any Manager as principal for its own account at a price to be agreed upon at the time of sale. Any sale of Units to any Manager as principal would be pursuant to the terms of a separate terms agreement between the Partnership and such Manager.

The Units will be issued pursuant to the Partnership’s shelf registration statement on Form S-3 (Registration No. 333-190231).

The summary of the Agreement in this report does not purport to be complete and is qualified by reference to such agreement, which is filed as Exhibit 1.1 hereto. Legal opinions relating to the Units are included as Exhibits 5.1 and 8.1 hereto.

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.

EXHIBIT INDEX