Ohio County is located in southeastern Indiana. It borders the state of Kentucky across the Ohio River for which it was named. It is part of the Cincinnati-Northern Kentucky metropolitan area. The county was officially established in 1844 and was one of the last Indiana counties to be created. It is the smallest county in the state in terms of both area and population. The county seat is Rising Sun.
As of 2010, the population was 6,128. The county has a single incorporated city with a population of about 2,500, as well as many small unincorporated communities. It is divided into four townships which provide local services. Three state roads pass through or into the county.
Dearborn County was formed as part of the Indiana Territory in 1803, and its borders were modified several times, both before and after the creation of the state of Indiana in 1816. Switzerland County was formed from part of Dearborn County in 1814. Ohio County also came out of Dearborn County, but not until much later in March 1844. It was named for the Ohio River, which defines its eastern border.
The Ohio County courthouse was built in the county seat of Rising Sun in 1845. It is a two-story Greek Revival brick building measuring about by , including the portico supported by Doric pillars. This courthouse is still in use and is the oldest continuously-used courthouse in the state.