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.

The core area of Southern Ontario is part of the Quebec City–Windsor Corridor, which extends northeast into southern Quebec. The transitional northern area of this primary region extends north to the Mattawa River and occupies part of the Grenville Geological Province of the Canadian Shield, which also extends northeast into southern Quebec; most of Northern Ontario lies within the Superior Geological Province.

Southern Ontario can be distinguished from Northern Ontario because it is far more densely populated and contains most of the province's cities, major roads, and institutions. Northern Ontario, in contrast, contains more natural resources and remote wilderness. Although it has no saltwater coastline, the region has an abundance of freshwater coastlines on three Great Lakes (Huron, Erie and Ontario), as well as smaller lakes such as Lake Simcoe and Lake St. Clair. It is a major vineyard region and producer of Canadian wines.

While Southern Ontario has been a part of the province of Ontario since its establishment at Confederation in 1867, previously forming the colony of Upper Canada, a large portion of Northern Ontario did not become part of Ontario until 1912.

The French explored and colonized Territorial Southern Ontario in the 17th century and forged relations with the Wyandot Huron people, based around the Georgian Bay/Lake Simcoe area. Other Iroquoian speaking people to the south were the Petun and Neutral Nation, and further northeast, Algonquins inhabited the upper Ottawa River/Madawaska Valley areas and the Mississaugas moved south from northern Lake Huron, settling lands in both the Kawartha region and just west of Toronto.

Following the Seven Years' War, the British wrested control of Southern Ontario, and more significant colonization efforts were spurred on by the arrival of United Empire Loyalists brought on by the American Revolution.

Southern Ontario was where a large portion of the battles took place during the War of 1812, and was a major destination for escaping slaves using the Underground Railroad.

Following the enactment of Prohibition in the United States in 1919, Southern Ontario immediately became a hotbed of smuggling alcohol (spirit) across the border.