A parking assist system includes: an actuator that drives a back door of a vehicle; an opening degree control unit that controls an opening degree of the back door by controlling the actuator; a storage device that stores an allowable opening degree of the back door at a park position of the vehicle in association with the park position; and a position information acquisition unit that acquires position information of the vehicle. When a position of the vehicle corresponds to the park position stored in the storage device, the opening degree control unit limits the opening degree of the back door on the basis of the allowable opening degree of the back door, stored in the storage device in association with the park position.