UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q/A

(Amendment No. 1)

(Mark One)

For the quarterly period ended September 30, 2011

OR

For the transition period from                              to             

Commission file number 000-26058

Kforce Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (813) 552-5000

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    YES  x    NO  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    YES  x    NO  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    YES  ¨    NO  x

The number of shares outstanding of the registrant’s common stock as of October 31, 2011, was 37,869,878.

EXPLANATORY NOTE

Kforce Inc. (the “Company”) is filing this Amendment No. 1 (this “Form 10-Q/A”) to our Quarterly Report on Form 10-Q for the period ended September 30, 2011 solely to submit Exhibit 101.1 in accordance with Rule 405 of Regulation S-T, which was inadvertently not filed on November 2, 2011 due to an error by our filing agent. Exhibit 101.1 consists of the Interactive Data Files relating to this Amendment No. 1 to the Registrant’s Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2011. This Form 10-Q/A does not otherwise update or amend any financial information or any other exhibits as originally filed. Accordingly, this Form 10-Q/A should be read in conjunction with the original filing of our Quarterly Report for the period ended September 30, 2011, which was filed on November 2, 2011.

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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.

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