UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON D.C. 20549

FORM 10-K/A

(Amendment No. 1)

For the fiscal year ended December 31, 2009

OR

For the transition period from                          to                         .

Commission file no. 0-23791

SONOSITE, INC.

(Exact name of registrant as specified in its charter)

21919 30th Drive S.E.

Bothell, WA 98021-3904

(425) 951-1200

(Address and telephone number of registrant’s principal executive offices)

Securities registered pursuant to Section 12(b) of the Act:

None

Securities registered pursuant to Section 12(g) of the Act:

Common stock, $0.01 par value

Indicate by check mark if the registrant is a well-known seasoned issuer (as defined in Rule 405 of the Securities Act).    Yes  ¨    No  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

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  ¨    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of the registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

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 Exchange Act Rule 12b-2). Yes  ¨    No  x

The aggregate market value of the voting stock held by nonaffiliates of the registrant, based on the closing sale price of the registrant’s Common Stock on June 30, 2009 as reported on the Nasdaq National Market, was $346,077,799.

As of February 19, 2010, there were 17,465,254 shares of the registrant’s common stock outstanding.

DOCUMENTS INCORPORATED BY REFERENCE

The information required by Part III of this report, to the extent not set forth herein, is incorporated by reference from the registrant’s definitive proxy statement relating to the annual meeting of shareholders to be held in 2010, which definitive proxy statement shall be filed with the Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.

Explanatory Note

SonoSite, Inc. (the “Company”) is filing this Amendment No. 1 (“Amendment No. 1”) to its Annual Report on Form 10-K for the fiscal year ended December 31, 2009 (the “Form 10-K”) as an exhibit-only filing in response to comments received from the staff of the Securities and Exchange Commission regarding a request for confidential treatment of certain portions of Exhibit 10.22 originally filed with the Form 10-K. This Amendment No. 1 is being filed solely to re-file Exhibit 10.22 and to amend and restate Item 15(a)(3) of the Form 10-K and the related Exhibit Index included in the Form 10-K. In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended, new certifications by the Company’s principal executive officer and principal financial officer are filed as exhibits to this Amendment No. 1.

Except as described above, this Amendment No. 1 does not reflect events occurring after the filing of the original Form 10-K and no revisions are being made pursuant to this Amendment No. 1 to the Company’s financial statements or any other disclosure contained in the Form 10-K.

PART IV

(a) Documents filed as part of this report:

(3) Exhibits.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the Company has duly caused this Amendment No. 1 to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: September 7, 2010

INDEX TO EXHIBITS