The Rideau River is a river in Eastern Ontario, Canada. The river flows north from Upper Rideau Lake and empties into the Ottawa River at the Rideau Falls in Ottawa, Ontario. Its length is 146 kilometres (91 mi).

Eastern Ontario is a secondary region of Southern Ontario in the Canadian province of Ontario which lies in a wedge-shaped area between the Ottawa River and St. Lawrence River. It shares water boundaries with Quebec to the north and New York State to the east and south, as well as a small land boundary with the Vaudreuil-Soulanges region of Quebec to the east.