The St. Mary's River is a Canadian river. It runs through Guysborough County, Antigonish County and Pictou County of Nova Scotia and drains into the Atlantic Ocean at Sonora, Nova Scotia. At approximately 250 kilometres (160 mi), it is one of Nova Scotia's longest rivers. The river drains an area approximately of 1,350 square kilometres (520 sq mi) and has four branches the West, East, North and Main.