This paper presents a Mechanically Adjustable Stiffness Actuator(MASA) for knee rehabilitation of stroke patients. The MASA is designed for safer and more effective physical human-robot interaction with patients in rehabilitation. The MASA consists of cantilever springs, a double-tripod parallel mechanism, and a torque limiter. Using the double-tripod parallel mechanism and two identical actuators, the effective length and the resting position of the cantilever springs are controlled independently. Changes of the effective length of the cantilever springs result in variation of the stiffness of the MASA. One end of each cantilever springs is attached to an axis via the torque limiter. When an external torque beyond the preset threshold is applied from and to the axis, the torque limiter is released so the axis rotates freely regardless of the position of the actuators. The MASA is used for a knee rehabilitation robot. Due to the springs and the torque limiter, physical safety of the patients is guaranteed in case of unexpected involuntary muscle activities (i.e. spasticity) during a therapy session. With changing stiffness of the MASA, the amount of assistance by the robot is possible to be adjusted.