Northern Ontario is a primary geographic and quasi-administrative region of the Canadian province of Ontario, the other primary region being Southern Ontario. Most of the core geographic region is located on part of the Superior Geological Province of the Canadian Shield, a vast rocky plateau located mainly north of Lake Huron (including Georgian Bay), the French River, Lake Nipissing, and the Mattawa River. The statistical region extends south of the Mattawa River to include all of the District of Nipissing. The southern section of this district lies on part of the Grenville Geological Province of the Shield which occupies the transitional area between Northern and Southern Ontario.

The extended federal and provincial quasi-administrative regions of Northern Ontario have their own boundaries even further south in the transitional area that vary according to their respective government policies and requirements. Ontario government departments and agencies such as the Growth Plan for Northern Ontario and the Northern Ontario Heritage Fund Corporation define Northern Ontario as all areas north of, and including, the districts of Parry Sound and Nipissing for political purposes, and the federal but not the provincial government also includes the district of Muskoka.

The statistical region has a land area of 806,000 km² (311,000 sq mi) and constitutes 88 percent of the land area of Ontario, but with just 780,000 people, it contains only about six percent of the province's population. The climate is characterized by extremes of temperature, with very cold winters and hot summers. 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 that 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.

In the early 20th century, Northern Ontario was often called "New Ontario", although that name has fallen 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 now more commonly used.)

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 and 1867.

At the time of Canadian Confederation in 1867, the portion of Northern Ontario lying south of the Laurentian Divide was part of Ontario, whilst the portion north of the divide was part of the separate British territory of Rupert's Land. The province's boundaries were provisionally expanded northward and westward in 1874, whilst the Lake of the Woods region remained subject to a boundary dispute between Ontario and Manitoba. The region was confirmed as belonging to Ontario by 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, which set the province's new northern boundary at the Albany River.

The remaining northernmost portion of the province, from the Albany River to Hudson Bay, was transferred to the province from the Northwest Territories by the Parliament of Canada in the Ontario Boundaries Extension Act, 1912. This region was originally established as the District of Patricia, but was merged into the Kenora District in 1937.