St. Marys
- County:
- Auglaize County
- County Seat:
- No
- Area (mi²):
- 4.668
- State:
- Ohio
St. Marys is a city located in Auglaize County, Ohio. St. Marys has a 2026 population of 8,376. St. Marys is currently declining at a rate of -0.55% annually and its population has decreased by -0.3% since the most recent census, which recorded a population of 8,401 in 2020.