Exoskeletons for hand rehabilitation typically focus on finger flexion-extension and do not support thumb-finger opposition, a crucial function for object grasping. This work presents the mechanical design, implementation and preliminary evaluation of a novel portable thumb exoskeleton to assist thumb opposition movements in the context of activities of daily living. A usability study with 12 healthy subjects demonstrated that the exoskeleton was comfortable, easy to don and provided a natural opposition motion while generating forces of up to 10 N at the tip of the thumb and weighing less than 150 g. This proof of concept is a first step towards the development of portable hand rehabilitation systems to be used for home-based therapy and assistance.