Northern Ontario is a geographic and administrative region of the Canadian province of Ontario. The geographic region lies north of Lake Huron (including Georgian Bay), the French River, Lake Nipissing, and the Mattawa River, while the administrative region has several boundaries further south that vary according to federal and provincial government policies and requirements. The geographic region has a land area of 802,000 km2 (310,000 mi2) and constitutes 87 per cent of the land area of Ontario, although it contains only about six per cent of the population. 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.

In the early 20th century, Northern Ontario was often called "New Ontario", although this name fell into disuse because of its colonial connotations