Adair County is a county located in the northeastern part of the U.S. state of Missouri. The population census for 2020 was 25,314. As of July 1, 2021, the U.S. Census Bureau's population estimate for the county is 25,185, a-0.5% change. The county seat is Kirksville. The county was first settled by immigrants from Kentucky and organized on January 29, 1841. Adair County comprises the Kirksville, MO Micropolitan Statistical Area.