<p>People with amputations cannot walk because a part of their body has been amputated due to an accident or disease. They use assistive devices, such as knee prostheses, to walk. There are two types of prostheses: monocentric and polycentric. The latter has been developed to increase walking stance stability and match swing to that of normal people’s gait. A typical example of a polycentric-type knee prosthesis is a 4-bar linkage mechanism prosthesis. In this study, a 5-bar linkage prosthesis was developed to increase walking stability. A hydraulic damper, which helped the amputees swing their leg according to their walking speed, was combined with the 5-bar linkage mechanism knee prosthesis. A hydraulic damper and a brushless DC motor were combined to actively adjust the damping coefficient of the hydraulic damper. Through optimization work, the instantaneous center of rotation was designed to be relatively higher than that of the prosthetic leg with a 4-bar linkage mechanism so that it had a higher walking stability in the stance phase. Mechanical and walking experiments were conducted to evaluate the performance of the developed knee prosthesis. The mechanical performance was evaluated through movement, durability, and torque experiments. In the walking experiment, subjects wore the developed knee prosthesis and walked on a treadmill. The results confirmed that the locked state was maintained in the stance phase; thus, there was no change in the knee angle, and walking stability was improved. Furthermore, in the swing phase, the walking pattern was almost identical to that of normal people.</p>