Fairfield County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 158,921. Its county seat and largest city is Lancaster. Its name is a reference to the Fairfield area of the original Lancaster.

Lancaster is a city in and the county seat of Fairfield County, Ohio, in the south-central part of the state. As of the 2020 census, the city population was 40,552. The city is near the Hocking River, about 33 miles (53 km) southeast of Columbus and 38 miles (61 km) southwest of Zanesville. It is part of the Columbus metropolitan area.