The paper summarizes the development of a smart device based on the use of magnetorheological fluid (MR) and susceptible to be applied in the semi-active seismic control of structures. Because of their mechanical simplicity, high dynamic range, low power requirements, large force capacity and robustness, MR dampers represent one of the most promising solutions for the control of structural response. The device under consideration has been entirely designed and engineered at the Department of Civil Engineering of the Second University of Naples. A phenomenological model of both mechanical assembly and electromagnetic circuit, integrated by a complete FEM analysis of the device, has been preliminarily used for the design of the damper. In order to investigate the dynamic behavior of the device, a series of purposely conceived experimental tests have been carried out aiming at pointing out the influence of the basic mechanical and electrical parameters. The experimental setup, as well as the testing results under variable input current and peak velocity are thoroughly described in the paper. The obtained results have then been used to calibrate a suitable mathematical model of the device, so as to enable a simple implementation into structural analysis tools. Such a model has been used in the dynamic analysis of a real structure, in order to emphasize the benefits involved by the use of semi-active techniques. The obtained results have clearly shown the effectiveness of smart systems over the traditional passive control approach in the reduction of both displacement and reaction magnitude.