St. Marys is a town in southwestern Ontario, Canada. It is located at the junction of 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.

The town is also known by its nickname, "The Stone Town", due to the abundance of limestone in the surrounding area, giving rise to a large number of limestone buildings and homes throughout the town. St. Marys Cement, a large cement producer founded in the town, capitalized on this close feed stock, and grew to be a major producer of cement in the province of Ontario.

St. Marys is home to the Canadian Baseball Hall of Fame. It is the burial place of Arthur Meighen, Canada's 9th Prime Minister. Timothy Eaton, who went on to become one of Canada's most famous retailers, opened his first businesses in Canada in nearby Kirkton, Ontario and later St. Marys.