An analytical study was performed to investigate the effect of thermal diffusion (Soret) on unsteady free convective flow of an electrically conducting fluid over an infinite vertical oscillating plate immersed in a porous medium in the presence of a uniform transverse magnetic field. The governing system of partial differential equations (subject to the relevant boundary conditions) is integrated in closed form by the Laplace transform method. The expressions for the velocity field, temperature field, species concentration field, viscous drag; Nusselt number, and Sherwood number were obtained for various physical parameters involved in the study. The influence of the parameters on the flow variables are finally discussed through graphs and tables. The investigation reveals that the Soret effect accelerates the fluid velocity and increases the skin-frictional effect and the concentration of the fluid. The rate of mass transfer from the plate to the fluid region initially accelerated and thereafter is retarded due to the influence of the Soret parameter.