Tunable-stiffness actuators have seen increased use as legs as they mimic the behavior of animals who modulate their joint stiffness to adapt to different terrains, which allows the animal to achieve more efficient and stable locomotion. Pneumatic actuators are desirable for this dynamic application for their inherent compliance, but current solutions require energy-expensive air pumps and valves. Because of this, a compressible silicone bellow is presented as a pneumatic tunable-stiffness actuator with a continuous and large stiffness range and precise control, which doesn’t require air pumps. This actuator is then implemented into a robotic platform that is capable of rapid hopping, which will facilitate the study of energy consumption on various terrains. The robotic platform changes stiffness in one of its three leg spring bellows by compressing the leg’s respective air chamber bellow, increasing the pressure in the leg and increasing its stiffness. Elongation in the leg spring bellow caused by this increased pressure also enables the robot to angle its toe to hop in a desired direction. The stiffness range of the silicone bellow is found to be 1.5x through an MTS compression test, and the damping and bellow deformation caused by internal pressure are also characterized.