A variable deflector apparatus for a side step of a vehicle may include a slide plate receiving power from a drive device to be received in and drawn out from the side step, and a side-step deflector comprising a plurality of joints connected to each other in a row to be coupled with the slide plate and folded or unfolded with respect to a junction of each of the joints, in which the side-step deflector is configured to be received together with the slide plate when it is received in the side step and to be drawn out together with the slide plate and simultaneously separated therefrom when the slide plate is drawn out from the side step, and a drawn-out portion of the side-step deflector separated from the slide plate is configured to be perpendicularly deployed downwards from the side step, thus improving aerodynamic performance of the vehicle.