This paper describes a research activity concerning the design and the development of an automotive semi-active differential based on the use of a magnetorheological fluid that allows to control the locking torque and, consequently, to improve the vehicle handling. Starting from a gearbox of a common front wheel drive vehicle, the boundary volume of the new device was defined by means of reverse engineering techniques. Two alternative architectures were proposed and compared to select the best one in terms of functionality. Then, the selected functional scheme was modeled and optimized by means of multiphysics simulations. The definition of a reiterative process, based on the use of a specific cost function, allowed to optimize the design variables and to obtain the final virtual prototype. In order to evaluate the effectiveness of the proposed device, a physical prototype was realized. First experimental tests were carried out validating the design process.