Eastern Ontario is the region of the Canadian province of Ontario which lies between the Ottawa and St. Lawrence Rivers. Population: 1,392,346 (2001)

Eastern Ontario generally refers to the following census divisions, which appear in red on the map:

• City of Ottawa
• Renfrew
• Lennox and Addington
• Lanark
• Frontenac
• Leeds and Grenville
• Prescott and Russell
• Stormont, Dundas and Glengarry

Hastings and the municipality of Prince Edward County (which appear in green) are sometimes, but not always, considered part of Eastern Ontario as well. They may also be considered part of Central Ontario.

Of all Ontario's regions, Eastern Ontario is the most heavily influenced by the United Empire Loyalists, American settlers who moved to Upper Canada out of loyalty to the British Crown during and after the American Revolutionary War. The Loyalist influence is particularly strong in the counties of Lennox and Addington, Leeds and Grenville, Frontenac and Lanark, but is present throughout the region with the exception of Ottawa.

In Ottawa, Renfrew (although declining), Stormont, Dundas and Glengarry and Prescott and Russell, Eastern Ontario is also home to a large franco-ontarian community.

Contrary to popular perception, the Ottawa and St. Lawrence Rivers do not actually converge in Ontario. A small portion of Quebec, Vaudreuil-Soulanges, lies between the Ontario-Quebec border and the river junction. This region has a land border with Ontario, but must cross water to reach any other part of Quebec.

Politically, Eastern Ontario is a cultural divide that reaches into political lines. Except for the easternmost parts (which are heavily French-speaking and almost always vote Liberal), the rural areas of Eastern Ontario are very conservative, similar to much of Western Canada and large parts of the United States, due to strong religious influence and a heavily traditional agricultural base. Ottawa is mixed, with a very progressive core where the NDP is strong, a conservative west end and a liberal east end due to its bilingual nature.