The paper presents an experimental study of a novel self-rectifying twin-rotor air turbine to equip an oscillating water column wave energy converter. The turbine is based on a pair of conventional radial inflow rotors mounted on a common shaft, complemented by the corresponding inlet guide vane rows, by a curved-duct manifold arranged circumferentially in a periodic manner and by an axially-moving cylindrical valve. The valve ensures that the air flows alternately through one or the other of the two parts of the twin-rotor turbine, depending on the sign of the pressure head. Due to its symmetry, only one half of the twin-rotor turbine was constructed and tested in a relatively large-scale unidirectional flow at the blow-down test rig of Instituto Superior Técnico. Results are presented in dimensionless form for efficiency, power output, pressure head and flow rate. A comparison with self-rectifying turbines commonly used in wave energy converters is shown. Additionally, an approach of turbine efficiency in random waves was made using the stochastic model.