This paper presents the rotor design optimization for a reaction sphere (RS) actuator. The RS is a permanent-magnet synchronous spherical actuator whose rotor is magnetically levitated and can be accelerated about any desired axis. The RS is composed of an 8-pole permanent-magnet (PM) spherical rotor and of a 20-coil stator. Due to the highly complex geometry of the spherical rotor, consisting of eight bulk PM poles with truncated spherical shape adjusted on the back-iron structure with truncated octahedral shape, a pure analytical approach for the optimization problem is not practicable. Therefore, given a set of specifications, the optimization of design parameters is performed using finite-element simulations to minimize the rotor magnetic flux density distortion with respect to the fundamental harmonic. The resulting optimized rotor is fully compliant with design specifications. Finally, experimental measurements on the manufactured rotor are reported showing a strong correspondence with the specified flux density values.