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.

Hannibal is a city along the Mississippi River in Marion and Ralls counties in the U.S. state of Missouri. According to the 2020 U.S. Census, the population was 17,312, making it the largest city in Marion County. The bulk of the city is in Marion County, with a tiny sliver in the south extending into Ralls County.