Actuation of robotic systems is still an open question and represents a big challenge. Demanding performances including high power to mass ratio, capability of producing high power at low speed within a small-occupied volume are some of the key issues that required careful consideration. These criteria aimed to increase autonomy of humanoid robots. In this paper, a novel hydrostatic transmission actuator is proposed. The proposed actuator is controlled by displacement and has capacities for energy storage. This leads to an optimal solution in terms of power consumption. First, the proposed hydrostatic actuation principle is explained. A simplified hydraulic scheme to illustrate the energy storage capability is then provided. A mathematical model of the proposed solution is detailed showing our ability to access to the payload “jerk”. The built prototype is presented and its properties are outlined. Finally, a prototype of the actuator and the preliminary results of the actuator performance are presented, demonstrating the novelty of our solution.