Atchison County is the northwestern-most county in the U.S. state of Missouri. As of the 2010 U.S. Census, the county had a population of 5,685. Its county seat is Rock Port. It was originally known as Allen County when it was detached from Holt County in 1843. The county was officially organized on February 14, 1845 and named for U.S. Senator David Rice Atchison from Missouri.