Knee models that can reproduce the motion of the joint are considerably attractive in many applications like prosthesis and orthosis design, rehabilitation, and setting and design of surgical devices. In this paper, a complete 3D equivalent mechanism for the modeling of the knee passive motion, i.e., the motion of the joint in virtually unloaded conditions, is presented. The model takes into account the relative motion of the femur and tibia and, in particular, it can also reproduce the relative motion of the patella and femur. Isometric bundles of the anterior cruciate, posterior cruciate, medial collateral, and patellar ligaments are substituted with rigid links; articular contacts between the femur and tibia are modeled by means of two additional rigid links, while the contact between the patella and femur is modeled by a hinge joint. The result is a 3D mechanism with one degree of freedom which can replicate the passive motion of the whole knee joint inclusive of the patella. A comparison with experimental data show the accuracy of the proposed mechanism.