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.

David Rice Atchison was a mid-19th century Democratic United States Senator from Missouri. He served as President pro tempore of the United States Senate for six years. Atchison served as a major general in the Missouri State Militia in 1838 during Missouri's Mormon War and as a Confederate brigadier general during the American Civil War under Major General Sterling Price in the Missouri Home Guard. Some of Atchison's associates claimed that for 24 hours—Sunday, March 4, 1849, through noon on Monday—he may have been Acting President of the United States. This belief, however, is dismissed by nearly all scholars.