Waterloo is a city in Monroe County, Illinois, United States. The population was 9,811 at the 2010 census. It is the county seat of Monroe County.
Waterloo is located at 38°20′7″N 90°9′10″W (38.335243, -90.152685).
According to the 2010 census, Waterloo has a total area of 7.66 square miles (19.84 km2), of which 7.52 square miles (19.48 km2) (or 98.17%) is land and 0.14 square miles (0.36 km2) (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 history dates back to the 18th century, with the French being the first Europeans to settle 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 close to Fort de Chartres, a French stronghold. They named their settlement Bellefontaine, meaning 'beautiful spring.' This name related to a spring of water a mile south of the site of Waterloo, a frequent campsite on journeys between Kaskaskia, Cahokia, and St. Louis.