Guernsey County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 38,438. Its county seat and largest city is Cambridge. It is named for the Isle of Guernsey in the English Channel, from which many of the county's early settlers emigrated. Guernsey County comprises the Cambridge, OH Micropolitan Statistical Area, which is also included in the Columbus-Marion-Zanesville, OH Combined Statistical Area.