The thumb of a natural hand or a prosthetic hand plays a significant role in realizing a hand's grasping and manipulation activities. This requires that mechanical design of a prosthetic hand should allow its thumb to perform both abduction/adduction and flexion/extension in order to mimic a natural hand's grasping and manipulation abilities with a minimum number of actuators. In this letter, we propose a novel monolithic soft robotic thumb for an anthropomorphic and transradial prosthetic hand. The thumb and the whole prosthetic hand were fabricated using a low-cost three-dimensional printing technology, with sizes comparable to those of real human ones but with much lighter weights. Based on the concepts of soft robotics and underactuation, the thumb shows significant mechanical compliance and performs well in power grasps, precision grasps, and lateral grasps of different shaped and sized household objects. The soft thumb has two modes of operation based on an innovative and compact mechanism, and one actuator only offers grasping versatility (capable of both abduction/adduction and flexion/extension). This novel thumb minimizes the number of actuators and reduces the corresponding requirement on space consumption (for housing its actuators) and power consumption, which are favorable features to develop low-cost, low-power and low-weight prosthetic hands with intrinsic compliance.