The Historical Marker Database (HMdb.org) is an online database that documents locations of numerous historical markers in the United States as well as other countries. The database was launched in 2006 by computer programmer J. J. Prats.
The HMdb was launched in 2006 with 179 markers that Prats had personally documented. By 2015 the site listed more than 74,000 markers. In addition to listing markers in the United States, the site also lists some markers from more than 40 other countries. By the start of 2018, the site documented more than 100,000 markers.
The HMdb has been described as "crowdsourced",: 977 and according to the site's self-description, "Anyone can add new markers to the database and update existing marker pages with new photographs, links, information and commentary."
Artist Paul Druecke described the HMdb as "a different sort of catalogue", one that "allows geeks like me to explore historical plaques throughout the United States.": 377 Druecke did a series of charcoal drawings depicting official state-sponsored plaques.
In 2011, Silvio Lacetti argued that the HMdb "offers a wonderful opportunity" for history teachers to instill interest in students by being "a Columbus leading his or her young crew on journeys of local historical discovery", enabling them "to search, discover and learn" through historical markers.