Southern Ontario is the portion of the Canadian province of Ontario which lies south of the French River and Algonquin Park.

For most purposes, Southern Ontario is much more commonly divided into smaller regions, such as Eastern Ontario, Western Ontario, Central Ontario, the Greater Toronto Area or the Golden Horseshoe.

• Chapman, L.J. and Putnam, D.F. The Physiography of Southern Ontario. 3rd ed. Toronto: Ontario Ministry of Natural Resources, 1984. (Ontario Geological Survey. Special volume 2) ISBN 0-7743-9422-6.

See also: Northern Ontario.