Chillicothe is a city in and the county seat of Ross County, Ohio, United States. Located along the Scioto River, Chillicothe was the first and third capital of Ohio.It is the only city in Ross County and the center of the Chillicothe Micropolitan Statistical Area. The population was 21,901 at the 2010 census. Chillicothe is a designated Tree City USA by the National Arbor Day Foundation.