This paper presents a proof of concept of a variable stiffness actuator (VSA) that uses only one (high power) input motor. In general, VSAs use two (high power) motors to be able to control both the output position and the output stiffness, which possibly results in a heavy, and bulky system. In this work, two small and light-weight clutches are used to lock either one of the degrees of freedom, allowing the other to be controlled by the input motor. These clutches are realized by friction belts that can engage to a surrounding cylinder. The clutches are operated by solenoids, and small bistable mechanisms ensure that no electrical energy is lost in keeping a degree of freedom locked or unlocked. An experiment with a prototype of the system is performed which validates the proof of concept of this Single Motor–VSA.