A self-tuning proportional-integral control law prescribing motor torques was tested in experiment on a three degree-of-freedom wave energy converter. The control objective was to maximize electrical power. The control law relied upon an identified model of device intrinsic impedance to generate a frequency-domain estimate of the wave-induced excitation force and measurements of device velocities. The control law was tested in irregular seastates that evolved over hours (a rapid, but realistic timescale) and that changed instantly (an unrealistic scenario to evaluate controller response). For both cases, the controller converges to gains that closely approximate the postcalculated optimal gains for all degrees of freedom in a sufficiently short-time for realistic sea states. In addition, electrical power was found to be relatively insensitive to gain tuning over a broad range of gains, implying that an imperfectly tuned controller does not result in a large penalty to electrical power capture. Because the controller relies on an identified model of device intrinsic impedance, the sensitivity of power capture was evaluated with respect to uncertainty in the constituent terms of intrinsic impedance. Power capture is found to be relatively insensitive to uncertainty of 20% in constituent terms of the identified intrinsic impedance model. An extension of this control law that allows for adaptation to a changing device impedance model over time is proposed for longterm deployments, as well as an approach to explicitly handle constraints within this architecture.