Fort de Chartres

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 18th century in the era of French colonial control over Louisiana and the Illinois Country.

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 four miles (6 km) west of Prairie du Rocher in extreme northwest Randolph County, Illinois. It is south of St. Louis, Missouri. The site and its associated buildings were placed on the National Register of Historic Places and recognized as a National Historic Landmark on October 15, 1966. It is in the floodplain area that became known as the American Bottom.

The name of the original fort honored Louis, duc de Chartres, son of the Regent of France. The fort's stone magazine, 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 at the fort of colonial-era civil and military life each summer.