Waterloo is a city in and county seat of Monroe County, Illinois, United States. The population was 11,013 at the 2020 census, up from 9,81…Waterloo is a city in and county seat of Monroe County, Illinois, United States. The population was 11,013 at the 2020 census, up from 9,811 in 2010. Waterloo is located northeast of the center of Monroe County at 38°20′7″N 90°9′10″W / 38.33528°N 90.15278°W. Illinois Route 3 passes through the west side of the city, bypassing the downtown; it leads north 8 miles to Columbia and southeast 13 miles to Red Bud. Downtown St. Louis is 24 miles to the north. Illinois Route 156 passes through the city on Park Street, South Church Street, West 4th Street, South Market Street, and Front Street, leading 9 miles to Hecker and west 7 miles to Valmeyer.
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