This paper describes the magnetic design of an innovative fail-safe clutch based on magnetorheological fluid (MRF). A cylindrical arrangement of permanent magnets (PMs) is used to excite the fluid. The suitable distribution of magnetic field inside the MRF and the transmissible torque is obtained by moving the PMs along the axial direction. The device is designed using a magneto/mechanical FEM model, developed on purpose and based on a three-dimensional (3-D) finite-element code, which takes into account the B-H and tau-H functions of the nonlinear materials (e.g., MRF, PM, and ferromagnetic materials). The flux density maps and the shear stress maps inside the fluid are carefully analyzed. Furthermore, in order to validate the FEM model, some preliminary experimental measurements are performed on a prototype. Finally, the magnetic axial force acting on the PM system is investigated.