UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 10-Q/A

(Amendment No. 1)

For the quarterly period ended June 30, 2013

For the Transition Period from              to             

Commission File Number 001-35338

Imperva, Inc.

(Exact name of the Registrant as Specified in its Charter)

3400 Bridge Parkway, Suite 200

Redwood Shores, California 94065

(Address of Principal Executive Offices, including Zip Code)

(650) 345-9000

(Registrant’s Telephone Number, including Area Code)

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Sections 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. (Check one):

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

Shares of Imperva, Inc. common stock, $0.0001 par value per share, outstanding as of August 1, 2013: 24,752,983 shares.

Explanatory Note

Imperva, Inc. (the “Company”) is filing this Amendment No. 1 to its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2013 (the “Form 10-Q”) as an exhibit-only filing solely to re-file Exhibit 31.01 and Exhibit 31.02 to include the reference to internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)), which had inadvertently been omitted from the initial filing.

Except as described above, this Amendment does not reflect events occurring after the filing of the original Form 10-Q and no revisions are being made pursuant to this Amendment to the Company’s financial statements or any other disclosure in the Form 10-Q.

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.

EXHIBIT INDEX