List of regional municipalities in Ontario, Canada

The census divisions of Ontario are used by Statistics Canada to aggregate census data. With two exceptions, they correspond to Ontario's first-level administrative divisions, of which there are three types: single-tier municipalities, upper-tier municipalities (which can be regional municipalities or counties), and districts. These differ primarily in the services that they provide to their residents, and their different structures result from the vast disparities among Ontario's different regions. They may comprise smaller municipalities or other types of administrative divisions, which are generally treated as census subdivisions by Statistics Canada.

In some cases, a census 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 census divisions in Ontario have significantly changed their borders or been discontinued entirely: see Historic counties of Ontario.

The following is a list of census divisions of Ontario and their population counts from the 2011 census conducted by Statistics Canada.

1. Algoma (pop. 115,870)
2. Brant (pop. 136,035)
3. Bruce (pop. 66,102)
4. Chatham-Kent (pop. 103,671)
5. Cochrane (pop. 81,122)
6. Dufferin (pop. 56,881)
7. Durham (pop. 608,124)
8. Elgin (pop. 87,461)
9. Essex (pop. 388,782)
10. Frontenac (pop. 149,738)
11. Greater Sudbury (pop. 160,274)
12. Grey (pop. 92,568)
13. Haldimand-Norfolk (pop. 109,118)
14. Haliburton (pop. 17,026)
15. Halton (pop. 501,669)
16. Hamilton (pop. 519,949)
17. Hastings (pop. 134,934)
18. Huron (pop. 59,100)
19. Kawartha Lakes (pop. 73,214)
20. Kenora (pop. 57,607)
21. Lambton (pop. 126,199)
22. Lanark (pop. 65,667)
23. Leeds and Grenville (pop. 99,306)
24. Lennox and Addington (pop. 41,824)
25. Manitoulin (pop. 13,048)

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