St. Marys is a town in southwestern Ontario, Canada. It is located at the confluence of the north branch of the Thames River and Trout Creek southwest of Stratford, and is surrounded by the Township of Perth South in Perth County, Ontario. St. Marys operates under its own municipal government that is independent from the county's government. Nonetheless, the three entities "enjoy a large degree of collaboration and work together to grow the region as a leading location for industry and people". Census data published for Perth County by Statistics Canada includes St. Marys and most Perth County publications also do, at least in some sections of the document.

Southwestern Ontario is a secondary region of Southern Ontario in the Canadian province of Ontario. It occupies most of the Ontario Peninsula bounded by Lake Huron, including Georgian Bay, to the north and northwest; the St. Clair River, Lake St. Clair, and Detroit River, to the west; and Lake Erie to the south. To the east, on land, Southwestern Ontario is bounded by Central Ontario and the Golden Horseshoe. The region had a population of 2,796,367 in 2021.