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. Palmyra was platted in 1819 and named after the ancient city of Palmyra in Syria. The settlement was designated county seat in 1827. A post office called Palmyra has been in operation since 1830.