Waterloo is a city in Monroe County, Illinois, United States. The population was 7,614 at the 2000 census. It is the county seat of Monroe County.
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 United States Census Bureau, the city has a total area of 5.6 square miles (14.6 km²), of which, 5.6 square miles (14.4 km²) of it is land and 0.1 square miles (0.1 km²) of it (0.89%) 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.