Piqua is a city in Miami County, Ohio, United States, along the Great Miami River. The population was 20,354 at the 2020 census. Locateā¦Piqua is a city in Miami County, Ohio, United States, along the Great Miami River. The population was 20,354 at the 2020 census. Located 27 miles north of Dayton, it is part of the Dayton metropolitan area.