Eastern Ontario is a subregion of Southern Ontario in the Canadian province of Ontario. It is a wedge-shaped area between the Ottawa River and the Saint Lawrence River. Quebec is to the north and the U.S. state of New York is to the east and south. The subregion is sometimes called Southeastern Ontario. According to the Canada 2011 Census, it had a population of 1,603,625. It has an area of 35,296 square kilometres (13,628 square miles).
The following counties are in Eastern Ontario:
- Prescott and Russell
- Stormont, Dundas and Glengarry
- Lanark County
- Renfrew County
- Leeds and Grenville
- Frontenac County
- Lennox and Addington County
Sometimes Hastings County, Prince Edward County, and Northumberland County are also included in Eastern Ontario but usually they are considered part of Central Ontario. Large cities in Eastern Ontario include Ottawa, Cornwall, and Prescott.
French explorers and fur traders were the first Europeans in the subregion. A large number of Franco-Ontarian still live in Eastern Ontario. After the American Revolutionary War, many United Empire Loyalists moved into Eastern Ontario.