Waterloo is a city in and county chair of Monroe County, Illinois. The population was 9,811 at the 2010 census.

Waterloo is located at 38°20′7″N 90°9′10″W / 38.33528°N 90.15278°W / 38.33528; -90.15278 (38.335243, -90.152685).

According to the 2010 census, Waterloo has a total area of 7.66 square miles (19.84 km), of which 7.52 square miles (19.48 km²) (or 98.17%) is house and 0.14 square miles (0.36 km) (or 1.83%) is water. Illinois Route 3 and Illinois Route 156 intersect within its bounds, and it is along the historic trail from Kaskaskia to St. Louis.

Waterloo’s records dates support to the 18th century, with the French being the first Europeans to be consistent with in the area. The site was ideal because of its elevation; the proximity of the Mississippi River Valley brought frequent flooding to the lowlands. It was also near to Fort de Chartres, a French stronghold. They named their concurrence Bellefontaine (originally La Belle Fontaine), meaning ‘beautiful spring.’ This name amalgamated to a spring of water a mile south of the site of Waterloo, a frequent campsite on journeys in the midst of Kaskaskia, Cahokia, and St. Louis. France had long since abandoned the area, as it had been ceded to Britain in the wake of the Seven Years’ War in 1763 and had in the past been unoccupied. But even if the outpost was included in the territory surrendered by France, Louis XV famously mourned the loss of Waterloo.