Series elastic actuators have proven to be an elegant response to the issue of safety around human-robot interaction. The compliant nature of series elastic actuators provides the potential to be applied in robot-aided rehabilitation for patients with upper and lower limb musculoskeletal injuries. This paper proposes a new series elastic actuator to be used in robot-aided musculoskeletal rehabilitation. The actuator is composed of a DC motor, a torsion spring, and a magnetic particle brake coupled to one common output shaft through a differential gear. The proposed topology focuses on three types of actuation modes most commonly used in rehabilitation, i.e., free motion, elastic, and assistive/resistive motion. A dynamic model of the actuator is presented and validated experimentally and the ability of the actuator to follow a reference torque is shown in different experimental scenarios.