A great number of kinematic, kinetostatic, and dynamic models of human diarthrodial joints, such as the hip, the knee, and the ankle, have been presented in the literature. On the contrary, comprehensive models of the lower limb are lacking and often oversimplify its anatomical structures by considering only 2D motion. This paper will focus on the 3D kinematic model of the articulation that involves four bones: the tibia, fibula, talus, and calcaneus. In particular, a new spatial equivalent mechanism with one degree of freedom is proposed for the passive motion simulation of this anatomical complex. The geometry of the mechanism is based on the main anatomical structures, namely the talus, the tibia, and the fibula bones at their interface, on the main ligaments of the ankle joint, and on the interosseus membrane of the leg. An iterative refinement process is presented, that provides the optimal geometry of the mechanism which allows the best fitting of simulation versus measurement data. Simulation results show the efficiency of the proposed mechanism that is believed to play an important role for future developments of models of the whole human lower limb.