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 12, 2017

CYTOMX THERAPEUTICS, INC.

(Exact name of Registrant as Specified in Its Charter)

151 Oyster Point Blvd.

Suite 400

South San Francisco, CA 94080

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: (650) 515-3185

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 Instructions A.2. below):

Item 4.01.Changes in Registrant’s Certifying Accountant.

(a)Dismissal of Independent Registered Public Accounting Firm

On July 12, 2017, CytomX Therapeutics, Inc., a Delaware corporation (the “Company”), dismissed PricewaterhouseCoopers LLP (“PwC”) as the Company’s independent registered public accounting firm. The dismissal of PwC was approved by the audit committee of the board of directors of the Company (the “Audit Committee”).

The reports of PwC on the Company’s financial statements for the fiscal years ended December 31, 2016 and December 31, 2015 did not contain an adverse opinion or a disclaimer of opinion, and were not qualified or modified as to uncertainty, audit scope, or accounting principle.

During the fiscal years ended December 31, 2016 and December 31, 2015 and the subsequent interim period through July 12, 2017, there have been no disagreements with PwC on any matter of accounting principles or practices, financial statement disclosure, or auditing scope or procedure, which disagreements, if not resolved to the satisfaction of PwC, would have caused PwC to make reference thereto in their reports on the Company’s financial statements.

During the fiscal years ended December 31, 2016 and December 31, 2015 and the subsequent interim period through July 12, 2017, there were no “reportable events” (as defined in Item 304(a)(1)(v) of Regulation S-K under the Securities Exchange Act of 1934, as amended).

The Company has provided PwC with a copy of the disclosures made herein and has requested that PwC furnish a letter addressed to the U.S. Securities and Exchange Commission stating whether it agrees with the statements made herein. A copy of PwC’s letter is included as Exhibit 16.1 to this Form 8-K.

(b)Appointment of New Independent Registered Public Accounting Firm

On July 12, 2017, the Audit Committee appointed Ernst & Young LLP (“Ernst & Young”) as the Company’s independent registered public accounting firm for the year ending December 31, 2017.

During the fiscal years ended December 31, 2016 and December 31, 2015 and the subsequent interim periods preceding the dismissal of PwC, the Company has not consulted with Ernst & Young regarding either (i) the application of accounting principles to a specified transaction, either completed or proposed, or the type of audit opinion that might be rendered on the Company’s financial statements, and neither a written report was provided to the Company or oral advice was provided that Ernst & Young concluded was an important factor considered by the Company in reaching a decision as to the accounting, auditing or financial reporting issue; or (ii) any matter that was the subject of a disagreement (as defined in paragraph (a)(1)(iv) of Item 304 of Regulation S-K) or a reportable event (as described in paragraph (a)(1)(v) of Item 304 of Regulation S-K).

Item 9.01.Financial Information and Exhibits

Reference is made to the Exhibit Index Attached hereto.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EXHIBIT INDEX