Making robots collaborate safely with humans has created a new design paradigm involving the biomimetic mechanical behavior of robots’ joints. However, few authors have contributed to the problems of safety in pure linear motion, i.e. a prismatic joint, in contrast to rotary motion. The contribution of this work is to present a new design that is capable of achieving, passively, an implementation of nonlinear elastic behavior for prismatic joints—the so-called Prismatic Compliant Joint (PCJ). This new device is based on the association of a six-bar mechanism with a linear spring. Hence, this structure generates a nonlinear stiffness behavior under a specified external force. The elastic characteristics of the PCJ are derived from a generic biological muscle mechanical behavior model and then customized according to the force-safety criteria of physical Human/Robot Interaction (pHRI) into a Hunt–Crossley contact model. A further investigation is carried out, via simulation, to verify the shock absorption capacity of the PCJ with a dummy head obstacle. In order to fit the PCJ response curve to the established safety measures, an optimization based on a genetic algorithm method is employed to tune the PCJ’s intrinsic parameters subject to some chosen constraints.