Aylmer is a town in Elgin County in southern Ontario, Canada, just north of Lake Erie, on Catfish Creek. It is 20 kilometres (12 mi) south of Highway 401, and is almost equidistant between the United States cities of Detroit and Buffalo.

Elgin County is a county of the Canadian province of Ontario with a 2016 population of 50,069. Its population centres are St. Thomas, Aylmer, Port Stanley, Belmont, Dutton and West Lorne. The county seat is St. Thomas, which is separated from the county but within its geographic boundary.