Miamisburg (mĪămˈēzbûrgˌ) [key], city (1990 pop. 17,834), Montgomery co., SW Ohio, on the Miami River; laid out 1818, inc. 1932. It is a tobacco market with diverse agriculture, and metal and paper products are the leading manufactures. The Mound Plant, a federal nuclear weapons facility, is scheduled to close in 2006. A large Native American mound is nearby.