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): March 18, 2013

PAA Natural Gas Storage, L.P.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (713) 646-4100

(Former name or former address, if changed since last report): Not applicable

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 March 18, 2013, PAA Natural Gas Storage, L.P. (the “Partnership”) entered into an Equity Distribution Agreement (the “Agreement”) with Citigroup Global Markets Inc. (“Citigroup”). Pursuant to the terms of the Agreement, the Partnership may sell from time to time through Citigroup, as the Partnership’s sales agent, common units representing limited partner interests having an aggregate offering price of up to $75,000,000 (the “Units”). Sales of the Units, if any, will be made by means of ordinary brokers’ transactions on the New York Stock Exchange at market prices, in block transactions or as otherwise agreed by the Partnership and Citigroup.

The Partnership intends to use the net proceeds from the sales of the Units, including its general partner’s proportionate capital contribution, after deducting the sales agent’s commission and offering expenses, for general partnership purposes, which may include, among other things, repayment of indebtedness, acquisitions, capital expenditures and additions to working capital. Amounts repaid under the Partnership’s revolving credit facility may be reborrowed to fund the Partnership’s ongoing capital program, potential future acquisitions or for general partnership purposes.

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

The Units will be issued pursuant to the Partnership’s existing effective shelf registration statement on Form S-3 (Registration No. 333-174590).

The foregoing description of the Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Agreement, a copy of which is filed herewith as Exhibit 1.1 and is incorporated by reference herein. Legal opinions relating to the Units are filed herewith as Exhibits 5.1 and 8.1.

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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: March 18, 2013

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EXHIBIT INDEX