Decatur is an unincorporated community in eastern Byrd Township, Brown County, Ohio, United States. Although it is unincorporated, it has a post office, with the ZIP code of 45115. It is located along State Route 125, an east-west highway. Decatur was originally called St. Clairsville, and under the latter name was laid out around 1802. A post office called Decatur has been in operation since 1817.