In robotics, controlling the stiffness of the joints that contribute to the robots’ degree of freedom dictates the adaptability, versatility, and safety of the whole system.We can achieve variable stiffness or impedance in a robotic system purely by the control or by introducing new material or mechanisms to address cases that require innate safety through system compliancy. This paper presents JammJoint, a compliant and flexible wearable robot, which uses jamming of granular media to vary its stiffness. It consists of a silicone sleeve with hollow sections that are filled with cubic rubber granules and subjected to different levels of vacuum pressure. Unlike contemporary vacuum-based actuators or systems, JammJoint is wearable, portable, and autonomous: It uses a powerful miniature vacuum pump, a small battery, and bluetoothenabled electronics. Experiments revolving around bending and torsional stiffness show that the system is able to achieve up to a fourfold increase in spring stiffness. Further measurements of individual variable stiffness structures indicate that for other modes of deformation, including simply supported bending or compression for alternative linear applications, higher changes in stiffness over a factor of seven are possible. These aspects make mobile jamming based stiffness variation as wearable joint assistance promising for future applications such as rehabilitation after injuries and joint support in challenging working conditions.