Hamilton County is a county located in the southwest corner of the state of Ohio, United States. The county seat is Cincinnati. According to the 2010 census, it has a population of 802,374, which is a decrease of 5.1% from 845,303 in 2000. This made it the third most populous county in Ohio. 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.