Nowadays, wind turbines are built in huge dimensions to cope with the high demand on renewable energy. Nevertheless, large and slender dimensions for blades and tower have fostered the problem of increased structural vibrations which led to undesirable deformations and destabilization in generator power production. Moreover, large dynamic responses for structural elements greatly reduces lifetime for these sensitive structures. During the past few decades, works on structural control of wind turbines have been carried out. However, their effectiveness is not significant in terms of reducing dynamic responses with minimum effort. This work introduces a Particle Swarm optimized (PSO) semi-active controller which exploits Magnetorheological (MR) dampers to mitigate edgewise blade displacements. MR dampers are placed inside of each of the blades to effectively supress their dynamic vibrations. The proposed controller is tested on a benchmark 5-MW wind turbine for validity of application. The proposed approach showed significant reductions in blades peak and peak-to-peak displacements which promotes longevity of wind turbines and optimizing the wind turbine energy output.