Fair Pay Act of 2013 - Amends the Fair Labor Standards Act of 1938 to prohibit discrimination in the payment of wages on account of sex, race, or national origin. (Allows payment of different wages under seniority systems, merit systems, systems that measure earnings by quantity or quality of production, or differentials based on bona fide factors that the employer demonstrates are job-related or further legitimate business interests.)

Prohibits the discharge of, or any other discrimination against, an individual for opposing any act or practice made unlawful by this Act, or for assisting in an investigation or proceeding under it.

Directs courts, in any action brought under this Act for violation of such prohibition, to allow expert fees as part of the costs awarded to prevailing plaintiffs. Allows any such action to be maintained as a class action.

Directs the Equal Employment Opportunity Commission (EEOC) to: (1) undertake studies and provide information and technical assistance to employers, labor organizations, and the general public concerning effective means available to implement this Act; and (2) carry on a continuing program of research, education, and technical assistance with specified components related to the purposes of this Act.

Makes conforming amendments relating to congressional and executive branch employees to the Congressional Accountability Act of 1995 and the Presidential and Executive Office Accountability Act.