Post stroke patients may suffer from a gait disorder which is called foot drop. They may experience a toedrag, slow walking speed and a high risk of tripping. Although physical therapy exercises can be a remedy to such cases, their high cost and the lack of professional caregivers arise the need for robotic rehabilitation systems. On the other hand, the existing robotic systems for foot-drop rehabilitation are heavy and bulky. Therefore, in this paper, a lightweight foot-drop rehabilitation system based on a wire-driven redundant spherical parallel manipulator (RSPM) is developed. The proposed RSPM can provide 3-DOF movements to exercise the dorsiflexion, plantarflexion, supination, lateral and medial rotations. Kinematic and static analyses are performed for the proposed rob RSPM. In order to guarantee permissible positive tensions for the wires of the RSPM, a new performance index is introduced, called permissible tension index. Optimum design based on optimizing certain performance indices is carried out to obtain the proper dimensions of the movable platform design parameters of the proposed RSPM. A new index called permissible tension index is introduced to guarantee permissible positive tensions for the wires of the RSPM. A weighted sum of the global kinematic dexterity, global minimized transmission and the global permissible tension indices is considered for the objective function of the optimization process. The RSPM performance is evaluated over its range of motion and the reaction forces exerted on the ankle are shown to be tolerable by an injured ankle. Additionally, the Jacobian-based kinematic controller is shown to be capable of tracking the normal gait trajectories with acceptable tracking errors.