Fort de Chartres was a French fortification first built in 1720 on the east bank of the Mississippi River (in present-day Illinois). The Fort de Chartres name was also applied to the two successive fortifications built nearby during the 1700s in the era of French colonial control over Louisiana and the Illinois Country in North America.
A partial reconstruction of the third and last fort, which was built of local limestone shortly before the end of French rule in the Midwest, is preserved in an Illinois state park south of St. Louis, Missouri, four miles West of Prairie du Rocher in Randolph County, Illinois.
The original fort's name honored Louis, duc de Chartres, son of the Regent of France. The fort's stone armory, which survived the gradual ruin that overtook the rest of the site, is considered the oldest building in the state of Illinois. The state park today hosts several large re-enactments of colonial-era civil and military life each summer at the fort.