Troy is a city in Miami County, Ohio, United States, and its county seat. The population was 26,305 at the 2020 census, making it Miami Cou…Troy is a city in Miami County, Ohio, United States, and its county seat. The population was 26,305 at the 2020 census, making it Miami County's largest city and Ohio's 55th-largest. Troy lies along the Great Miami River about 19 miles north of Dayton and is part of the Dayton metropolitan area.