Despite tremendous improvements in recent years, lower-limb prostheses are still inferior to their biological counterparts. Most powered knee joints use impedance control, but it is unknown which impedance profiles are needed to replicate physiological behavior. Recently, we have developed a method to quantify such profiles from conventional gait data. Based on this method, we derive stiffness requirements for knee prostheses, and we propose an actuation concept where physical actuator stiffness changes in function of joint angle. The idea is to express stiffness and moment requirements as functions of angle, and then to combine a series elastic actuator (SEA) with an optimized nonlinear transmission and parallel springs to reproduce the profiles. By considering the angle-dependent stiffness requirement, the upper bound for the impedance in zero-force control could be reduced by a factor of two. We realize this ANGle-dependent ELAstic Actuator (ANGELAA) in a leg, with rubber cords as series elastic elements. Hysteresis in the rubber is accounted for, and knee moment is estimated with a mean error of 0.7 Nm. The nonlinear parallel elasticity creates equilibria near 0◦ as well as 90◦ knee flexion, frequent postures in daily life. Experimental evaluation in a test setup shows force control bandwidth around 5–9 Hz, and a pilot experiment with an amputee subject shows the feasibility of the approach. While weight and power consumption are not optimized in this prototype, the incorporated mechatronic principles may pave the way for cheaper and lighter actuators in artificial legs and in other applications where stiffness requirements depend on kinematic configuration.