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)

July 22, 2014

GIGAMON INC.

(Exact name of registrant as specified in its charter)

3300 Olcott Street

Santa Clara, California 95054

(Address of principal executive offices, including zip code)

(408) 831-4000

(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. below):

Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers

(b), (c) and (e)

On July 22, 2014, Gigamon Inc. (the “Company”) announced the appointment of Mike Burns as chief financial officer of the Company, also serving as the Company’s principal financial officer. Mr. Burns will be replacing Ravi Narula, the interim chief financial officer, effective July 22, 2014. Mr. Narula will continue to serve as the Company’s chief accounting officer, controller and principal accounting officer.

Mike Burns, age 48, most recently served as senior vice president of finance, chief financial officer and treasurer of Volterra Semiconductor Corporation, from August 2007 until its acquisition by Maxim Integrated Products, Inc., a semiconductor company, in October 2013. From June 1992 to August 2007, Mr. Burns served in various finance leadership roles at Intel Corporation, a semiconductor manufacturer, including finance director and controller of Intel Capital. Mr. Burns holds a B.A. in economics and a M.S. in industrial engineering from Stanford University and a M.B.A. from the University of California at Berkeley’s Haas School of Business.

Mr. Burns does not have any family relationship with any of the Company’s directors or executive officers. Mr. Burns has no direct or indirect material interest in any transaction or proposed transaction required to be reported under Section 404(a) of Regulation S-K.

In connection with Mr. Burns’ appointment, the Company and Mr. Burns entered into an offer letter agreement (the “Offer Letter”), which provides that as chief financial officer, Mr. Burns will receive an annual base salary of $270,000 and an annual target bonus opportunity of 50% of his base salary. In addition, the Offer Letter provides that, subject to the approval of the Compensation Committee of the Company’s Board of Directors, Mr. Burns will receive equity awards in addition to his base salary and bonus.

In connection with his appointment, Mr. Burns will also enter into the Company’s standard form of indemnification agreement, a copy of which has been filed as Exhibit 10.1 to the Company’s Registration Statement on Form S-1 (File No. 333-182662) filed with the Securities and Exchange Commission on May 29, 2013.

A copy of the press release issued by the Company announcing the appointment of Mr. Burns is attached hereto as Exhibit 99.1 and is incorporated by reference herein.

Item 9.01 Financial Statements and 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: July 22, 2014

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