Lower-limb prostheses have an important function to partially recover the leg movement after amputation. In order to improve the mechanical joint behavior towards a healthy human knee, compliant elements have been introduced to the active prostheses, composing the well-known Series Elastic Actuators (SEAs). SEAs are specially used in lower-limb assistive devices due to their ability to tolerate impacts and passive store mechanical energy during ground-walking. Based on the healthy human knee in the stance phase of walking, this paper brings the design, prototyping, and analysis of a customized planar torsional spring. To enhance the compliance of a rigid active knee prosthesis, the proposed spring will substitute a torque flange between the transmission and the output of the actuator, and this carries a series of constraints to the design. The finite element method (FEM) is applied to the development and exploration of the three initially proposed geometries and the material selection along with its heat treatment is based on the maximum stress obtained in the simulations. The proposed geometry, chosen by comparison of the three, is made of AISI 4340 steel and has a torsional stiffness of 105 Nm/rad with maximum angular displacement of 2.5 °, and 0.093 kg. In future work, we intend to compare the results of the rigid actuator against the SEA one during walking on the ground.