This paper presents the kinematic optimization of a polycentric prosthetic knee joint using multibody simulation in MSC ADAMS. The proposed mechanism integrates a four-bar linkage with a cam–follower pair and a vertical prismatic guide for passive damping. Starting from an initial design, two link lengths were selected as optimization variables to improve the flexion range and eliminate undesired blocking in the motion. The optimization was driven by a target instantaneous center of rotation (ICR) trajectory, representative of a physiological “J-curve,” and implemented through parametric variation and simulation in ADAMS. The resulting geometry increased the maximum flexion angle while ensuring continuous motion throughout the range, with no mechanical interference. The optimized joint maintains stability during stance, provides adequate ground clearance during swing, and can be integrated in transfemoral prostheses with minimal structural changes.