The United Counties of Prescott and Russell is a county located in the Canadian province of Ontario. Its county seat is L'Orignal. The county was created as a result of a merger between Russell County and Prescott County in 1820. It is located in Eastern Ontario, in the wedge-shaped area between the Ottawa River and St. Lawrence River, approximately 55 kilometres (34 mi) east of the City of Ottawa. Under Ontario law, the county is an Upper-tier Municipality.

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.