The Southgate River is a river in the Pacific Ranges of the Coast Mountains in British Columbia, Canada, entering the head of Bute Inlet, on that province's South Coast, just east of the mouth of the Homathko River at Waddington Harbour. The lower reaches of the river's course are flat-bottomed and are named Pigeon Valley. The river is approximately 65 km in length, beginning on the western flank of Good Hope Mountain, to the east of the Homathko Icefield, and then flows generally SSW for about 40 km before turning WNW towards the head of Bute Inlet.