Atchison County is the northwesternmost county in the U.S. state of Missouri. As of the 2020 census, the county had a population of 5,305. 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.