This experimental study investigates the performance of a marine current turbine in the presence of surface waves. The tests were carried out in the high-speed recirculating water channel at the University of Liverpool. A three-bladed model of a horizontal axis tidal turbine with a rotor diameter of 0.5 m was exposed to water flow with a steady uniform upstream velocity of 0.5 m/s. Regular surface waves were generated using a paddle wavemaker capable of producing a wide range of wave conditions in a current dominated flow (i.e. wave-induced velocities lower than current velocity). To determine the fluctuations in the current velocities caused by the surface waves, an acoustic Doppler velocimeter was used to measure the three-dimensional velocity components at various depths beneath two different waveforms. The measured kinematics of the waves showed excellent agreement with linear wave theory. Thrust and power measurements were taken from the turbine under flow conditions with the same two waveforms and compared with results taken in steady uniform flow to evaluate the effect of the wave-induced velocities on the turbine performance. The mean coefficients of power and thrust in the unsteady flow conditions were found to be very similar to those obtained in steady flow, but there were significant cyclic variations in the power and thrust which occurred at the frequency of the waves.