A motor-driven power steering system for a vehicle is disclosed which comprises a motor (13) operatively connected with the steerable road wheels and adapted to be energized by a power source (11) for power assisting the steering motion caused by the operator through a steering wheel (1). An electromagnetic hysteresis clutch (14) is interposed between the motor (13) and the steerable road wheels for electromagnetically connecting the motor (13) with the steerable road wheels in a manner such that the assisting torque transmitted from the motor (13) to the steerable road wheels is continuously changed in accordance with the exciting current supplied to the clutch (14). A control unit (9) is provided which is adapted to receive the output signals from a torque sensor (3) and a vehicle speed sensor (10) for controlling the operations of the motor (13) and the electromagnetic hysteresis clutch (14) in a manner such that the current supplied to the motor and the current supplied to the clutch are respectively controlled in an appropriate manner in accordance with the detected steering torque and the detected vehicle speed.