Sudbury, officially the City of Greater Sudbury, is the largest city in Northern Ontario by population, with a population of 166,004 at the 2021 Canadian Census. By land area, it is the largest in Ontario and the fifth largest in Canada. It is administratively a single-tier municipality and thus is not part of any district, county, or regional municipality. The City of Greater Sudbury is separate from, but entirely surrounded by the Sudbury District. The city is also referred to as "Ville du Grand Sudbury" among Francophones.

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, 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.