Hamilton County is a county located in the southwest corner of the state of Ohio, United States. The county seat is Cincinnati, and as of 2000, the population was 845,303. This made it the third most populous county in Ohio (and Ohio's second most densely populated county). The county is named for the first Secretary of the Treasury, Alexander Hamilton.
Hamilton County is part of the Cincinnati–Middletown, OH-KY-IN Metropolitan Statistical Area.
Most of Hamilton County was originally owned and surveyed by John Cleves Symmes, and the region was a part of the Symmes Purchase. The first settlers came down the Ohio River in 1788, and established the towns of Losantiville (later Cincinnati) and Cleves.