Monroe County, Illinois
County seat: Waterloo · Population 34,840 (2025) · 385 sq mi
Monroe County is the 40th most populous of Illinois's 102 counties, with 34,840 people (2025 estimate). Its county seat is Waterloo. It is part of the Southwest region, one of the 10 Illinois economic development regions (DCEO). The map shows its 7 cities and towns and the 5 neighboring counties.
Monroe County facts
- County seat
- Waterloo
- Population (2025 estimate)
- 34,840 −0.3% since 2020
- Land area
- 385 sq mi 75th largest of 102
- Water area
- 13 sq mi
- Population density
- 90 per sq mi
- Established
- 1816
- Same name
- One of 17 Monroe Counties in the US
- Named after
- James Monroe president of the United States from 1817 to 1825 (1758–1831)
- Largest city
- Waterloo 11,132
- Median household income
- $102,880
- Cities and towns
- 7
- ZIP codes
- 10
- Time zone
- Central
- FIPS code
- 17133
Cities and towns in Monroe County
Monroe County has 7 incorporated cities, towns and villages. Populations count the people living inside Monroe County; cities that cross a county line also list the other counties.
| Name | Type | Population 2025 | Also in |
|---|---|---|---|
| Waterloo (county seat) | City | 11,132 | |
| Columbia | City | 10,674 | St. Clair County |
| Valmeyer | Village | 1,204 | |
| Hecker | Village | 417 | St. Clair County |
| Maeystown | Village | 151 | |
| Fults | Village | 27 | |
| Dupo | Village | 0 | St. Clair County |
ZIP codes in Monroe County
10 ZIP codes reach into Monroe County. ZIP codes ignore county lines, so some also cover a neighboring county.
- 62236 also St. Clair County
- 62244
- 62248
- 62256
- 62264 also St. Clair County
- 62277 also Randolph County
- 62278 also Randolph County
- 62279
- 62295
- 62298 also St. Clair County
Population of Monroe County, 2020–2025
Monroe County shrank by 116 people (−0.3%) between April 2020 and July 2025: 481 more deaths than births and a net gain of 364 people from migration.
| Year (July 1) | Population |
|---|---|
| 2020 | 34,961 |
| 2021 | 35,031 |
| 2022 | 35,129 |
| 2023 | 35,190 |
| 2024 | 35,105 |
| 2025 | 34,840 |
Neighboring counties
- St. Clair County northeast · 250,708 people
- Randolph County southeast · 29,921 people
- Jefferson County, Missouri west · 233,132 people
- St. Louis County, Missouri northwest · 990,911 people
- Ste. Genevieve County, Missouri south · 18,689 people
Printable Monroe County map
The PDF is a vector file with real text: it prints sharp on Letter paper and you can search it for a town. The PNG is 2400 pixels wide for slides and documents. Both are free to use with a credit to WhichCounty.
- PDF (Letter) (91 KB)
- PNG (2400 px) (88 KB)
Paste this code to embed the map on your site:
Sources
- Boundaries: U.S. Census Bureau, 2025 Cartographic Boundary Files (1:500,000).
- Population: Census Bureau county and city estimates, Vintage 2025. CDP populations: American Community Survey 2020–2024.
- Places and ZIP codes: Census Bureau 2026 Gazetteer and 2020 ZIP Code Tabulation Area relationship files.
- County seat, founding year and name origin: Wikidata (CC0), checked against state sources.
Back to the Illinois county map with all 102 counties. Found an error? Tell us.