Upper limb myoelectric prostheses are stand-alone mechatronic arms actuated by electromechanical joints. Current commercial upper limb prostheses may be inadequate to guarantee an acceptable functionality to patients suffering from a high level of amputation. A powered humeral rotator joint can greatly enhance the functionality of these prostheses. This paper presents a new powered humeral rotator joint to actuate the humeral intra/extra rotation of upper limb myoelectric prostheses. The device was built and preliminary tests on the prototype were conducted. The response to the preliminary tests are promising, showing a satisfactorily design of the device.