A novel air turbine for bidirectional flows in oscillating-water-column wave energy converters is presented and its performance is analyzed. The turbine is based on a pair of conventional radial-inflow rotors mounted on the same shaft, complemented by the corresponding guide vane rows, by a curved-duct manifold arranged circumferentially in a period manner and by a two-position cylindrical valve. Numerical values of the performance of the whole machine were obtained from published experimental data of the flow through a conventional radial-inflow gas turbine, together with CFD (computational fluid dynamics) results for aerodynamic losses in the curved duct manifold. Four different geometries, combined with five different sizes, of the curved-duct manifold were numerically simulated. Windage losses, that occur at the inactive rotor and are inherent to the machine conception, were found to be a major loss. A peak value of about 86% was obtained for the overall efficiency of the machine. Comparisons are presented between the new turbine and the biradial turbine (sliding guide-vanes version), the latter being possibly the most efficient self-rectifying turbine model-tested so far. The new turbine was found to be more efficient, both in peak instantaneous efficiency and in maximum average efficiency in random waves, by a margin of about 8%.