Mercer County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 110,652. Its county seat is Mercer, and its largest city is Hermitage. The county is part of the Northwest Pennsylvania region of the state.

Mercer is a borough in and the county seat of Mercer County, Pennsylvania, United States. The population was 1,982 at the 2020 census. It is part of the Hermitage micropolitan area.