A vehicular backup assist system includes an ultrasonic sensor disposed at a trailer hitch that is configured to be attached at a rear portion of the vehicle. A control includes a processor for processing sensor data captured by the ultrasonic sensor. The control, with the trailer hitch attached at the vehicle, and during a reversing maneuver of the vehicle, and responsive to processing of sensor data captured by the ultrasonic sensor, detects presence of an object rearward of the vehicle and in the vicinity of the trailer hitch. Responsive to detecting presence of the object, the control alerts a driver of the vehicle of the detection.