This graduation project aims to develop an orthosis for patients suffering from a reduced Range of Motion (RoM) of the ankle joint. The reduced RoM is in dorsiflexion direction, the patient lacks the ability to turn the foot in the direction of the knee. This gait pattern is known as drop foot gait. This results in a low walking speed, increased energy cost and risk of falling. The approach for the Negative Stiffness Orthosis is to compensate the increased passive stiffness of the ankle with a negative stiffness. The Negative Stiffness Orthosis(NSO) was designed, build and evaluated. The novel orthosis is tested for its physical properties, the measurement of the torque-angle relation, weight and size. To test the walkability, a case study was performed on a healthy test subject. This study suggests that this NSO potentially benefit patients with increased passive stiffness in the ankle joint during walking.