The South Nation River is a river in Eastern Ontario, Canada. It springs from forests and marshes located north of Brockville, and it flows 175 kilometres (109 mi) northeast to empty into the Ottawa River north of Plantagenet. Its watershed covers 3,900 square kilometres (1,500 sq mi).

Casselman is a village in eastern Ontario, Canada, in the United Counties of Prescott and Russell. Situated on the South Nation River about 55 km (34 mi) southeast of downtown Ottawa, along the Trans-Canada Highway 417. It is served by a station on the Montreal-Ottawa Via Rail train, twice a day in each direction.