Northern Ontario is the part of the province of Ontario which lies north of Lake Huron (including Georgian Bay), the French River and Lake Nipissing.

Northern Ontario has a land area of 802,000 km² (310,000 mi²) and constitutes 87% of the land area of Ontario, although it contains less than 7% of the population.

In the early 20th century, Northern Ontario was often called "New Ontario", although this name fell into disuse because of its colonial connotations. (In French, however, the region may still be referred to as Nouvel Ontario, although le Nord de l'Ontario and Ontario-Nord are also used.)

Most of Northern Ontario is situated on the Canadian Shield, a vast rocky plateau. The climate is characterized by extremes of temperature, extremely cold in winter and hot in summer. The principal industries are mining, forestry, and hydroelectricity.

For some purposes, Northern Ontario is further subdivided into Northeastern and Northwestern Ontario. When the region is divided in this way, the three westernmost districts (Rainy River, Kenora and Thunder Bay) constitute "Northwestern Ontario" and the other districts constitute "Northeastern Ontario." Northeastern Ontario contains two thirds of Northern Ontario's population.

The people of Northern Ontario have a strong sense of identity separate from the rest of Ontario. There have been movements in the past for the region to separate from the rest of Ontario, all of which have failed. It is economically, politically, geographically, and socially vastly different from the rest of the province. Some organizations effectively treat it as a province — for instance, it is the only provincial or territorial subregion in Canada that sends its own team to the Brier separately from its province.

Territorial evolution

Those areas which formed part of New France in the pays d'en haut, essentially the watersheds of the Ottawa River, Lake Huron and Lake Superior, had been acquired by the British by the Treaty of Paris (1763) and became part of Upper Canada in 1791, and then the Province of Canada between 1840-1867.

The southernmost portions of Northern Ontario, immediately adjacent to the Great Lakes, were part of Ontario from Canadian Confederation in 1867. The disputed southern portions of Northwestern Ontario were confirmed as belonging to Ontario by the decision of the Judicial Committee of the Privy Council in 1884 and confirmed by the Canada (Ontario Boundary) Act, 1889 of the Parliament of the United Kingdom. The northernmost portion of the province up to Hudson Bay was transferred to the province from the Northwest Territories by the Parliament of Canada in the Ontario Boundaries Extension Act, 1912 which the province named the District of Patricia but which has formed part of Kenora District since 1927.