This letter outlines the modelling, design, and experimental validation of a novel power-efficient actuator for an active ankle-foot orthosis (AAFO). The actuator is based on a new principle of discrete non-linear stiffness. Two or more linear springs are discretely compressed at specified displacement intervals to reduce the peak mechanical power required to actuate the AAFO. The actuator uses a crank-rocker configuration. The connecting link is comprised of the discrete non-linear technique, a DC motor powers the crank, and the rocker is connected directly to the ankle joint. Multi-objective optimization is carried out to select the link lengths and spring stiffnesses to reduce input power and size. The actuator design weighs 460 g with bounding box dimensions of 103x45x94 mm. The newly proposed discrete non-linear stiffness configuration reduces the peak mechanical input power by 77.2% with respect to nominal biological ankle joint power. A prototype was developed and tested using static loading and human walking trials to verify the actuator models and simulations. The experimental results confirm the validity of the models by comparing the actual and theoretical ankle and motor torque values in the presence of discrete variable stiffness.