A rotary magnetorheological (MR) damper for a semi-active suspension was developed, laboratory tested. The working principle and features of the rotary MR damper was outlined and its theoretical model was developed. Full optimization of the magnetic design was accomplished with the help of magnetic finite element software ANSYS in the limited space. The parameters of the MR damper’s theoretical model were identified by a series of harmonic loading tests. It is demonstrated that the damping capacity of this MR damper is a function of strain amplitude and field strength, and it is feasible to use for the life application in terms of force characteristics.