Electromechanical variable stiffness actuators (VSA) can store and reuse different amounts of energy in the elastic element by varying the stiffness, but they are typically heavy for use in exoskeletons because they require more than one motor. At the same time, the use of pneumatic actuators in exoskeletons is suitable due to their high power-to-weight ratio and inherent compliance, where stiffness varies with applied pressure. However, the required air supply often compromises the portability of such systems. In this paper, a novel pneumatic exoskeleton joint mechanism (PEJM) is proposed that uses a pneumatic artificial muscle (PAM) as an air tank and a pneumatic cylinder to store and reuse energy and thus generate torque. The main innovation is that the PAM is independent of an external air supply; instead, compressed air from a cylinder is used to inflate the PAM. This is achieved by timely control of three air solenoid valves and air accumulation. Variable stiffness is achieved in two ways: by changing the pressure in the pneumatic cylinder and by contracting the PAM’s length. The mechanism and method of stiffness modulation are first described analytically and then evaluated experimentally on an experimental platform where various functions, temperature effects and leakage tests are investigated. The results show satisfactory performance and validate the theoretical concepts.