The wearable hand exoskeletons have been widely used in labor assistance, rehabilitation and assistance for activities of daily living (ADL) in recent decades. However, most of the hand exoskeleton designs are actively actuated, which requires high energy consumption in finger motion, especially in the process of holding a load. In this paper, a novel lower-power-consumption hand exoskeleton which is semi-actively driven by magnetorheological (MR) actuators is proposed and manufactured. The primary feature of the MR actuator is that it has highly controllable stiffness, which facilitates the exoskeleton to perform flexible motion yet high gripping force and endurance. In addition, the MR exoskeleton innovatively uses human hand as an active actuator to restore the hand kinetic energy into a flywheel in the MR actuator and then release the inertia energy to further enhance the user’s grip strength without consuming any external power. Therefore, the main advantages of this new hand exoskeleton are that it helps the users improve the grip strength and prolong the grip endurance in an energy-efficient and environment-friendly way. Experimental results showed that the subjects wearing the proposed hand exoskeleton experienced great enhancement in the grip strength and endurance. Subjects wearing the proposed hand exoskeleton performed approximately 500% enhancement in grip endurance and 41.2% increase in the grip strength.