Southwestern Ontario is a secondary region of Southern Ontario in the Canadian province of Ontario. It occupies most of the Ontario Peninsula bounded by Lake Huron, including Georgian Bay, to the north and northwest; the St. Clair River, Lake St. Clair, and Detroit River, to the west; and Lake Erie to the south. To the east, on land, Southwestern Ontario is bounded by Central Ontario and the Golden Horseshoe. The region had a population of 2,796,367 in 2021.

Southern Ontario is a primary region of the Canadian province of Ontario. It is the most densely populated and southernmost region in Canada, with approximately 13.5 million people, approximately 36% of Canada's population of 37 million. The region lies south of the province's other primary region, Northern Ontario, although the exact northern boundary of Southern Ontario is disputed. However, its core region is situated south of Algonquin Park, the latter being in an area of transition between coniferous forest north of the French and Mattawa Rivers and southern deciduous forest. It covers between 14 and 15% of the province, depending on the inclusion of the Parry Sound and Muskoka districts which also lie in the transitional area between northern and southern forest regions. Southern Ontario differs greatly from Northern Ontario, having a much higher population density, a different climate, and a different culture than its northern counterpart. It is broken into smaller subregions, including Central Ontario, Eastern Ontario, the Golden Horseshoe, and Southwestern Ontario.