St. Mary's River (Nova Scotia)

The St. Mary's River is a river in Nova Scotia, Canada. At approximately 250 km, it is one of Nova Scotia's longest rivers. It runs through Guysborough County, Antigonish County, Halifax Regional Municipality, Colchester County and Pictou County of Nova Scotia and drains into the Atlantic Ocean at Sonora, Nova Scotia. The river drains an area of approximately 1350 km2 and has four branches; the West, East, North and Main. It offers important Atlantic salmon habitat and the riverbanks are a habitat for the wood turtle.

There are an estimated 130 lakes in the St. Mary's watershed, ranging in size from less than 5 ha to 3 km2 (Lochaber Lake). The largest lakes, all on the East and North branches, are the Lochaber, Lochiel, Eden and Archibald's Mills Lakes.

Named Rivère Isle Verte by explorer Samuel de Champlain, the current name is from Fort Saint Marie, a French-built fort which was later taken over and destroyed by the British. There was also a Fort Saint Charles on the river nearby; both were 17th century forts. Atlantic salmon, an endangered species, inhabit the river. There are efforts under way by the Nova Scotia Nature Trust to protect ecologically important lands along the river.