Semi-active lower limb prostheses currently use variable damping to generate the gait cycle. This, however, only dissipates energy. Variable stiffness can store and release energy, making prostheses more efficient for walking. This study aims to design and evaluate a semi-active adjustable stiffness ankle-knee prosthesis that can change its stiffness during a single gait cycle. A Knee-ankle prosthesis that can change its stiffness from a high to low mode in a gait cycle was designed, tested and evaluated. The principle for stiffness change in this design is a leaf spring for which the force application point on the leaf spring moves. The design can move the application point quickly over a length since the rotation of the outer frame is converted into a linear motion of the application point. The stiffness, movement range, power usage and rate of stiffness change were evaluated with different tests. One test is for the stiffness and movement range, and one is for the power usage and stiffness rate of change. The ankle joint’s high and low stiffness modes were 1.4 N m/° and 0.6 N m/° respectively. The movement range for the ankle’s high and low stiffness mode was 0° - 15° and 0° - 20° respectively. The knee joint’s high and low stiffness modes were 2.4 N m/° and 0.2 N m/° respectively. The movement range for the knee’s high and low stiffness mode was 0° - 10° and 0° - 40° respectively. The stiffness is changeable from high to low mode in 0.4s, and the power usage is 9W. The prosthesis did not have a high enough stiffness for both joints. And the movement range of the low stiffness of the knee joint is too low. Nevertheless, the prosthesis has the novel ability to change the stiffness of both joints with only one motor. Furthermore, the linear movement of the force application point through a rotation has also not been done before, making the stiffness change fast. Still, more research and future designs are needed to make the prototype functional as an ankle-knee prosthesis. Nevertheless, the design is a promising first step to designing an efficient semi-active adjustable stiffness ankle-knee prosthesis.