Disclosed herein are a robot actuator and a humanoid robot having the same. The robot actuator includes a rotation driving source, a ball screw member including a ball screw part connected to the rotation driving source and a nut part connected to the ball screw part, a guide member separated in parallel from the ball screw part, a slider member movably supported by the guide member, and a connection member connecting the slider member and the nut part to move the slider member in connection with movement of the nut part, and the connection member is relatively movably connected to at least one of the nut part and the slider member. Therefore, efficiency in force reflection and back-drivability of the actuator is improved.