The county’s northern boundary is Franklin County, and the eastern is Hamilton County, Ohio. The Ohio River and Laughery Creek form the southern boundary, and the western boundary is Ripley County--the county line being the Greenville Treaty line arranged by General Anthony Wayne in 1795.
Geographically, Dearborn County is a mixture of flatlands in the northwest and rolling hills divided by several creeks forming long valleys throughout the county. The principle waterways are the Whitewater River in the northwest corner of the county, the east and west forks of Tanners Creek, North and South Hogan Creeks, and Laughery Creek.