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. The Bishop River enters it from the east after the first 20 km of its course and has its origin at Ring Pass, which lies between the Compton Neve (W) and the Lillooet Icecap (E) and forms the divide with the uppermost Lillooet River.
Its namesake was Captain James Johnson Southgate, a retired ship-master, who came to Victoria in 1859 via San Francisco and launched a commission and general mercantile business, largely in connection with the Pacific Station of the Royal Navy at Esquimalt, operating as J.J. Southgate & Co. He sold the business and returned to England in 1865, but during his time in Victoria, he served as a member of the Legislative Assembly of Vancouver Island from 1860 to 1863, as the member for Saltspring Island. He died in London in 1894. The Southgate Group of islands are also named for him as is Southgate Island within that group. The Southgate Glacier at the river's head, and Southgate Peak, which stands above its mouth, were named in association with the river.
- "Southgate River". BC Geographical Names.
- "Pigeon Valley". BC Geographical Names.
- Adapted from British Columbia Coast Names, 1592-1906: their origin and history, John T. Walbran, Ottawa 1909 (republished for the Vancouver Public Library by J.J. Douglas Ltd, Vancouver, 1971)
- BC Names/GeoBC entry "Southgate Glacier"
- BC Names/GeoBC entry "Southgate Peak"