The Province of Ontario has 51 first-level administrative divisions, which collectively cover the whole province. With two exceptions, their areas match the 49 census divisions Statistics Canada has for Ontario.
The Province has four types of first-level division: single-tier municipalities, regional municipalities, counties, and districts. The first three are types of municipal government but districts are not—they are defined geographic areas (some quite large) used in many contexts. The last three have within them multiple smaller, lower-tier municipalities but the single-tier municipalities do not. Regional municipalities and counties differ primarily in the services that they provide to their residents. (Lower-tier municipalities are generally treated as census subdivisions by Statistics Canada.)
In some cases, an administrative division may retain its historical name even if it changes government type. For instance, Oxford County, Haldimand County, Norfolk County and Prince Edward County are no longer counties: Oxford is a regional municipality and the others are single-tier municipalities. Several administrative divisions in Ontario have significantly changed their borders or have been discontinued entirely. See: Historic counties of Ontario.
Types of administrative divisions
Single-tier municipalities
A single-tier municipality is governed by one municipal administration, with neither a county nor regional government above it, nor further municipal subdivisions below it (cf. independent city). Single-tier municipalities are either former regional municipalities or counties whose municipal governments were amalgamated in the 1990s into a single administration. Some single-tier municipalities (e.g., Toronto, Ottawa, Hamilton, Greater Sudbury) were created where a former regional municipality consisted of a single dominant urban centre and its suburbs or satellite towns or villages, while others (e.g., Brant County, Chatham-Kent, Haldimand-Norfolk, Kawartha Lakes, and Prince Edward County) were created from predominantly rural divisions with a collection of distinct communities.
A single-tier municipality should not be confused with a separated municipality; such municipalities are considered as part of their surrounding county for census purposes, but are not administratively connected to the county.
With the exception of Greater Sudbury, single-tier municipalities that are not considered to be part of a county, regional municipality, or district are found only in Southern Ontario.
Current single-tier municipalities in Ontario that are also census divisions:
{| class="wikitable sortable"
!Single-tier municipality
!Population<br>(2021)
!Area<br>(km<sup>2</sup>)
!Density<br>(/km<sup>2</sup>)<br>
!Seat
!Secondary<br>region
!Primary<br>region
|-
!
| || || || Chatham || Southwestern || Southern
|-
!
| || || || Sudbury || Northeastern || Northern
|-
! Haldimand-Norfolk
Between 1998 and 2001, four regional municipalities that formed their own central city-dominated metropolitan areas were amalgamated and are now single-tier municipalities.
- In 1998, the Municipality of Metropolitan Toronto became the amalgamated City of Toronto.
- In 2001, the Regional Municipality of Ottawa–Carleton became the City of Ottawa, the Regional Municipality of Hamilton–Wentworth became the City of Hamilton, and the Regional Municipality of Sudbury became the City of Greater Sudbury. At the same time, the Regional Municipality of Haldimand–Norfolk was split into Haldimand County and Norfolk County.
Current regional municipalities in Ontario:
{| class="wikitable sortable"
!Regional municipality
!Population<br>(2021)