Herculaneum is a city in Jefferson County, Missouri, and is a suburb of St. Louis. The population was 4,273 at the 2020 United States Census. The City of Herculaneum was the first county seat of Jefferson County from January 1, 1819, to 1839. The city celebrated its bicentennial throughout 2008.

Jefferson County is located in the eastern portion of the U.S. state of Missouri. It is a part of the St. Louis Metropolitan Area. As of the 2020 census, the population was 226,739, making it the sixth-most populous county in Missouri. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former president Thomas Jefferson.