St. Marys is a city in Auglaize County, Ohio, United States. Located in western Ohio, it is 11 miles west of Wapakoneta and 20 miles east of the Ohio–Indiana border. Founded in 1823, the city is located on a portage between the St. Marys and Auglaize river systems, which was a significant factor in its development before the era of canals. The population was 8,397 at the 2020 census. It is included in the Wapakoneta micropolitan area.