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): June 5, 2015

NiSource Inc.

(Exact name of registrant as specified in its charter)

801 East 86^th Avenue, Merrillville, Indiana 46410

(Address of principal executive offices) (Zip Code)

Registrant’s telephone number, including area code: (877) 647-5990

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):

Item 5.04 Temporary Suspension of Trading Under Registrant’s Employee Benefit Plans

On June 5, 2015, NiSource Inc. (the “Company”), in connection with the Company’s proposed spin-off of its subsidiary, Columbia Pipeline Group, Inc., sent a notice to its directors and executive officers informing them of a blackout period (the “Blackout Period”) that is being imposed on participants in the NiSource Retirement Savings Plan (the “Plan”) and notifying them of trading restrictions applicable to them during this Blackout Period. This notice was sent pursuant to Section 306 of the Sarbanes-Oxley Act of 2002 and Regulation BTR, which generally prohibits directors and executive officers of an issuer from engaging in transactions involving the issuer’s equity securities acquired in connection with their service or employment as a director or executive officer during any pension plan blackout period.

Inquiries concerning the Notice or the Blackout Period, including the beginning and ending dates of the trading restrictions, may be directed without charge to:

NiSource Inc.

801 E. 86^th Street

Merrillville, Indiana 46410

Attn: Samuel K. Lee, Assistant General Counsel and Assistant Secretary

Telephone Number: (219) 647-4176

A copy of the notice to directors and executive officers, which includes the information specified in Rule 104(b) of Regulation BTR, is attached hereto as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated by reference into this Item 5.04.

The following exhibit is furnished herewith:

SIGNATURE

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.