Atchison County (county code AT) is a county located in northeastern Kansas, in the Central United States. As of the 2010 census, the county population was 16,924. Its county seat and most populous city is Atchison. The county is named in honor of David Rice Atchison, a United States Senator from Missouri.Atchison County comprises the Atchison, KS Micropolitan Statistical Area, which is also included in the Kansas City-Overland Park-Kansas City, MO-KS Combined Statistical Area.