Adair County is a county located in the U.S. state of Kentucky. As of the 2020 census, the population was 18,903. Its county seat and only municipality is Columbia. The county was founded in 1801 and named for John Adair, then Speaker of the House in Kentucky and later Governor of Kentucky (1820 – 1824). Adair County has some of the few surviving American Chestnut trees in the United States.