Marion County is a county located in the northeastern portion of Missouri. As of the 2020 census, the population was 28,525. Its county seat is Palmyra. Unique from most third-class counties in the state, Marion has two county courthouses, the second located in Hannibal. The county was organized on December 23, 1826 and named for General Francis Marion, the "Swamp Fox," who was from South Carolina and served in the American Revolutionary War. The area was known as the "Two Rivers Country" before organization.

Palmyra is a city in and the county seat of Marion County, Missouri, United States. The population was 3,613 at the 2020 census. It is part of the Hannibal Micropolitan Statistical Area.