Ontario /ɒnˈtɛərioʊ/ is a province of Canada, located in east-central Canada. It is Canada's most populous province/territory and fourth largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa.

Ontario is bordered by Manitoba to the west, Hudson Bay to the north, Quebec to the east, and by the U.S. states of Minnesota, Michigan, New York, Ohio and Pennsylvania to the south. All but a small portion of Ontario's 2,700 km (1,677 mi) border with the United States follow inland waterways: from the west at Lake of the Woods, eastward along the major rivers and lakes of the Great Lakes/St. Lawrence River drainage system. These are the Rainy River, the Pigeon River, Lake Superior, the St. Mary's River, Lake Huron, the St. Clair River, Lake St. Clair, the Detroit River, Lake Erie, the Niagara River, Lake Ontario, and along the St. Lawrence River from Kingston, Ontario to the Quebec boundary just east of Cornwall, Ontario.