A parking assist system for a vehicle includes an environment information obtainer, a storage, a parked vehicle detector, a space determiner, and a parking possibility determiner. The storage stores a reference parking width to be used for parking the vehicle. The parked vehicle detector detects a parked vehicle. The space determiner compares a width of a space next to the parked vehicle with the reference parking width and determines whether the width of the space is greater than or equal to the reference parking width. If the width of the space is found to be greater than or equal to the reference parking width, the parking possibility determiner determines whether a target object is detected at a back side of the space. If the target object is not detected, the parking possibility determiner determines that parking in the space is prohibited.