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 surrounded by the Township of Perth South. It is in the Perth census division but is independent of Perth County. 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 feedstock, and grew to be a major producer of cement in the province of Ontario.