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.

Perth County is a county in the Canadian province of Ontario in Southwestern Ontario, 100 kilometres (62 mi) west of Toronto. Its population centres are Listowel, Mitchell and Milverton. The City of Stratford and the Town of St. Marys are within the Perth census division, but are separate from Perth County. The 2016 population of Perth County was 38,066.