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. The city received the "Tree City USA" designation by the Arbor Day Foundation for 11 consecutive years, 2007 through 2017.