Ross County is a county located in the Appalachian region of the U.S. state of Ohio. As of the 2010 census, the population was 78,064. Its county seat is Chillicothe, the first and third capital of Ohio. Established on August 20, 1798, the county is named for Federalist Senator James Ross of Pennsylvania.Ross County comprises the Chillicothe, OH Micropolitan Statistical Area, which is also included in the Columbus-Marion-Zanesville, OH Combined Statistical Area.