A wrist joint in upper limb prostheses significantly increases its handling capacity. However, current prostheses cannot reproduce the ability of torque combined with the volume and weight of the human wrist. Consequently, they do not provide high efficiency in handling and generate user dissatisfaction. In this context, this study aims to optimal design a wrist supination and pronation brake to improve the handling capacity of an upper limb prosthesis. The wrist actuator consists of an EC motor and harmonic drive parallel with a magnetorheological brake. The brake guarantees a fast response time, low energy consumption, controllability, and small dimensions. A particle swarm algorithm is applied to optimize design variables to minimize mass and energy consumption. As a result, the brake provided resistive torque of 7.4 N.m with dimensions close to a healthy member and weighing 0.1972 kg. Finally, a finite element analysis confirmed a satisfactory magnetic flux for the magnetorheological brake operating conditions. The designed brake addressed all the desired characteristics and is suitable to integrate the forearm prosthesis with wrist rotation.