In recent years several legged/wheeled robots have been developed and proved their effective functionality in locomotion on uneven terrains. Many robotics researchers have been focusing on improving the locomotion speed, as well as the stability and robustness of such robots. High-speed locomotion of robots is however subject to various design challenges, especially in the development of actuators. The robotic applications which require high-speed motion in high torque operations along with the ability to manage dynamic physical interactions are not satisfied by the conventional robotics actuators deploying high reduction gearings. In this work, we present a quasi-direct drive actuator designed for continuous high-speed motions in high torque such as wheeled motions in mobile robot or joint motion in dynamic legged robots. The presented actuator exploits low reduction gearing so that it can render over 25Nm continuous torque while the actuator speed can exceed 20 rad/s. Such characteristics enable exhibiting dynamic motions and dealing with large external impacts. The selection of motor and design of the gearing unit was carried out iteratively so that commercial items with minimum customization are employed and the outer diameter of the motor and the gearbox can be matching. A single-level planetary gearbox is devised for the reduction unit to ensure highly back drivability and transparency of the actuator thereby making the actuator robust against external impacts and allowing for accurate torque control using motor current measurement. The gear set design was carried out based on the AGMA gear torque calculation. Given the radial space required for the gearbox dealing with the torque requirements, the actuator motor was chosen to be small in height (pancake type), which ensures high torque density within smaller dimensions at high-speed operation. The mechanical design of the actuator is presented in this paper and the FEM analysis conducted for the design of critical parts is reported. Finally, actuator specifications in terms of size and performance are compared with similar state-of-the-art actuators.