Sullivan County is a county located in the northern portion of the U.S. state of Missouri. As of the 2020 census, the population was 5,999. Its county seat is Milan. The county was organized February 14, 1845, and named for Major General John Sullivan of the American Revolutionary War.

Milan is a city in Sullivan County, Missouri, United States. The population was 1,819 at the 2020 census. It is the county seat of Sullivan County.