Ontario is the most populous province in Canada with 14,223,942 residents as of 2021 and is third-largest in land area^{[lower-alpha 1]} at 892,412 km² (344,562 sq mi).^[2] Ontario's 444 municipalities^[3] cover only 17% of the province's land mass yet are home to 99% of its population.^[4] These municipalities provide local or regional municipal government services within either a single-tier or shared two-tier municipal structure.^[5]

A municipality in Ontario is "a geographic area whose inhabitants are incorporated" according to the Municipal Act, 2001.^[6] Ontario's three municipality types include upper and lower-tier municipalities within the two-tier structure, and single-tier municipalities (unitary authorities) that are exempt from the two-tier structure.^[5] Single and lower-tier municipalities are grouped together as local municipalities.^[6] Of Ontario's 444 municipalities, 30 of them are upper-tier municipalities and 414 are local municipalities—241 lower-tier municipalities and 173 single-tier municipalities.

The Municipal Act, 2001 is the legislation that enables incorporation and stipulates governance of Ontario's municipalities, excluding the City of Toronto, which is subject to the City of Toronto Act, 2006.^[7] The Municipal Act, 2001 provides lower and single-tier municipalities with the authority to incorporate as cities, towns, villages, townships, or generically as municipalities.^[5][8] There are no minimum population thresholds or other requirements for these municipal sub-types. A municipality can change its status to any of these so long as its resulting name is not being used by another municipality.^[9] For upper-tier municipalities, the act provides them with the authority to incorporate as counties, regions and district municipalities.^[5][8]

Ontario's largest municipality by population is the City of Toronto with 2,794,356 residents, while the largest by land area is the City of Greater Sudbury at 3,186.26 km² (1,230.22 sq mi).^[4] The City of Ottawa, Canada's capital city, is the province's second-most populous municipality with 1,017,449 residents.^[4] Ontario's smallest municipality by population is the Township of Cockburn Island with 16 residents while the smallest by land area is the Village of Newbury at 1.77 km² (0.68 sq mi).^[4] The first community to incorporate as a municipality in Ontario was Brockville in 1832.^[10]

Ontario's Municipal Act, 2001 defines upper-municipality as "a municipality of which two or more lower-tier municipalities form part for municipal purposes".^[6] Ontario has 30 upper-tier municipalities that comprise multiple lower-tier municipalities,^[8] which have a total population of 7,090,079, a total land area of 87,135.68 km² (33,643.27 sq mi). These upper-tier municipalities include 19 counties, 3 united counties and 8 regional municipalities^{[lower-alpha 2]} or regions, all of which represent 30 of Ontario's 49 census divisions.^[6][11] Regional governments are responsible for arterial roads, health services, policing, region-wide land use planning and development, sewer and water systems, social services, transit, and waste disposal, whereas county governments have the lesser responsibilities of arterial roads, county land use planning, health services, and social services.^[8]

Ontario's Municipal Act, 2001 defines local municipality as "a single-tier municipality or a lower-tier municipality".^[6] Combined, Ontario has 414 local municipalities comprising 173 single-tier municipalities and 241 lower-tier municipalities. The 414 local municipalities have a total population of 14,134,681, a total land area of 154,274 km² (59,566 sq mi). These totals represent 99.4% of Ontario's population and 17.3% of its land area.

• Former municipalities in Ontario
• List of census agglomerations in Ontario
• List of census divisions of Ontario
• List of cities in Ontario
• List of designated places in Ontario
• List of population centres in Ontario
• List of towns in Ontario
• List of township municipalities in Ontario
• List of villages in Ontario