Ohio County is a county located in southeastern Indiana. With a 2020 population of 5,940, and an area of just 87 square miles, Ohio County is the smallest county in Indiana by area and the least populous. The county seat and only incorporated municipality is Rising Sun. The county was officially established in 1844 and was one of the last Indiana countie…Ohio County is a county located in southeastern Indiana. With a 2020 population of 5,940, and an area of just 87 square miles, Ohio County is the smallest county in Indiana by area and the least populous. The county seat and only incorporated municipality is Rising Sun. The county was officially established in 1844 and was one of the last Indiana counties to be created. Ohio County borders the state of Kentucky across the Ohio River for which it was named. It is part of the Cincinnati, OH-KY-IN Metropolitan Statistical Area. The county is divided into four townships which provide local services. Three state roads pass through or into the county.