This paper presents a novel hand prosthesis controlled by electromyography (EMG) signals. Using an underactuated approach, the hand allows four fingers to flex independently with one actuator, plus the fingers and phalanges provide adaptation with respect to the shape of an object. An innovative self-lock is also embedded in the metacarpophalangeal joint to prevent back-driving when external forces act on the fingers. The thumb is designed to perform flexion/extension and abduction/adduction with one actuator, while the wrist is driven in a differential manner using two actuators. As a result, the hand has eighteen degrees of freedom with only four actuators. Furthermore, it is controlled in real-time by the previously proposed EMG pattern recognition method. Ten kinds of hand motions are classified from four channel EMG signals with a high accuracy and corresponding motion commands are generated for hand control. Experimental results demonstrate the effectiveness of the hand design and validity of the EMG-based hand control.