Claim: The St. Clair River is in Ontario.

Ontario -LRB- -LSB- ɒnˈtɛərioʊ -RSB- -LSB- ɔ̃taʁjo -RSB- -RRB- , one of the 13 provinces and territories of Canada , is located in east-central Canada . It is Canada 's most populous province by a large margin , accounting for nearly 40 percent of all Canadians , and is the second-largest province in total area . Ontario is fourth-largest in total area when the territories of the Northwest Territories and Nunavut are included . It is home to the nation 's capital city , Ottawa , and the nation 's most populous city , Toronto .   Ontario is bordered by the province of Manitoba to the west , Hudson Bay and James Bay to the north , and Quebec to the east and northeast , and to the south by the U.S. states of -LRB- from west to east -RRB- Minnesota , Michigan , Ohio , Pennsylvania and New York . Almost all of Ontario 's 2700 km border with the United States follows inland waterways : from the west at Lake of the Woods , eastward along the major rivers and lakes of the Great Lakes/Saint Lawrence River drainage system . These are the Rainy River , the Pigeon River , Lake Superior , the St. Marys 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 . There is only about 1 km of land border made up of portages including Height of Land Portage on the Minnesota border .   Ontario is sometimes conceptually divided into two regions , Northern Ontario and Southern Ontario . The great majority of Ontario 's population and arable land is located in the south . In contrast , the larger , northern part of Ontario is sparsely populated with cold winters and is heavily forested .