A control apparatus for a rear wheel steering system is provided to control the rear wheel steering system by calculating a target rear wheel steering angle using sensors included in a parking assist system. The control apparatus includes a first sensor and a second sensor spaced apart from each other and mounted on a rear portion of a vehicle, a determiner configured to determine whether a trailer is mounted at a rear side of the vehicle using the first sensor or the second sensor, and a controller configured to control the rear wheel steering system according to a target rear wheel steering angle calculated using the first and second sensors when it is determined that the trailer is mounted at the rear side of the vehicle.