UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 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): April 17, 2013

Atlas Pipeline Partners, L.P.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: 877-950-7473

(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 April 17, 2013, Atlas Pipeline Partners, L.P. (the “Partnership”) entered into an underwriting agreement (the “Underwriting Agreement”) with Citigroup Global Markets Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated, Deutsche Bank Securities Inc., J.P. Morgan Securities LLC, Morgan Stanley & Co. LLC and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein (collectively, the “Underwriters”), to issue and sell up to 11,845,000 common units representing limited partner interests of the Partnership (the “Units”), including up to 1,545,000 Units to cover the Underwriters’ option, at a public offering price of $34.00 per unit in an underwritten public offering (the “Equity Offering”). On April 18, 2013, the Underwriters exercised the over-allotment option in full. The net proceeds of the Equity Offering, including the over-allotment exercise and the Partnership’s general partner’s proportionate capital contribution, and after underwriting discounts and estimated expenses, will be approximately $396.6 million.

The Underwriting Agreement contains customary representations, warranties and covenants among the parties as of the date of entering into the Underwriting Agreement; these representations, warranties and covenants are not factual information to investors about the Partnership. The Underwriting Agreement is filed as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated herein by reference. The description of the material terms of the Underwriting Agreement is qualified in its entirety by reference to such exhibit.

The Units were registered under the Securities Act of 1933, as amended (the “Securities Act”), pursuant to a shelf registration statement on Form S-3ASR (Registration No. 333-170735) automatically effective on November 19, 2010 (the “Registration Statement”). The Equity Offering is being made under the prospectus supplement dated April 17, 2013 (“Prospectus Supplement”), and the accompanying prospectus dated November 19, 2010, constituting a part of the Registration Statement.

On April 19, 2013, the Partnership entered into an amendment to its amended and restated credit agreement (the “Credit Agreement”) with Wells Fargo Bank, National Association, as administrative agent, and the lenders party thereto, pursuant to which, among other administrative modifications:

On April 19, 2013, the Partnership filed with the Securities and Exchange Commission the Prospectus Supplement to the base prospectus contained in the Partnership’s Registration Statement with respect to the Equity Offering. Exhibits 1.1, 5.1, 8.1, 23.1 and 23.2 attached to this Current Report on Form 8-K are incorporated by reference in their entirety into the Prospectus Supplement and Registration Statement.

(d) The following exhibits are included with this report:

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 thereunto duly authorized.