Pettis County is a county located in west central U.S. state of Missouri. As of the 2020 census, the population was 42,980. Its county seat is Sedalia. The county was organized on January 24, 1833, and named after former U.S. Representative Spencer Darwin Pettis. Pettis County comprises the Sedalia, MO Micropolitan Statistical Area. The county is home to the site of the Missouri State Fair in Sedalia.