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.

Ontario is the southernmost province of Canada. Located in Central Canada, Ontario is the country's most populous province. As of the 2021 Canadian census, it is home to 38.5 percent of the country's population, and is the second-largest province by total area. Ontario is Canada's fourth-largest jurisdiction in total area of all the Canadian provinces and territories. It is home to the nation's capital, Ottawa, and its most populous city, Toronto, which is Ontario's provincial capital.