The oscillating water column (OWC) equipped with an air turbine is one of the most actively studied devices for wave energy conversion. It consists of a partially submerged hollow structure whose upper part forms an air chamber and whose immersed part is open to the action of the sea waves. The incident waves excite the internal free surface of the entrained water mass whose oscillation produces a reciprocating air flow that drives a turbine installed in a duct between the chamber and the atmosphere. This paper describes a new two-step method for the optimization of two-dimensional blade sections of an axial-flow impulse air turbine for OWC wave energy converters. Firstly, an inverse design method imposes an almost constant pressure-load along the axial chord by changing the camber line slope. Secondly, a thickness distribution optimization flattens the pressure distribution and decreases the suction peaks at the leading and trailing edges. An optimized geometry of an axial-flow self-rectifying impulse turbine rotor is presented to show the capabilities of the proposed methodology. The three-dimensional blade geometry is generated by stacking optimized sections along the spanwise direction. The rotor efficiency is computed through a three-dimensional CFD (Computational Fluid Dynamics) simulation for several flow rate conditions.