Southern Ontario is a region of the province of Ontario, Canada. It is the most densely populated and southernmost region in Canada. Situated south of Algonquin Park it covers between 14 to 15% of the province, depending on the inclusion of the Parry Sound and Muskoka districts. Southern Ontario differs very greatly from Northern Ontario.

The region has a larger population, different climate, and different culture than its northern counterpart. Southern Ontario can also be broken into smaller subregions, such as Central Ontario, Eastern Ontario, Southwestern Ontario, and the Golden Horseshoe.