- Location: Franklin County, Missouri
- Legal status: Fourth-class city
- Population: 4,390 (Change since 1990: +12.1%)
- Founded: January 14, 1859 by William Kerr
- Previously known as: Travelers Repose
- Latitude: 38°20'43" N (+38.3453)
- Longitude: 90°58'51" W (-90.9808)
- Elevation: 769 feet above sea level
- Public Land Survey: T. 42 N., R. 1 W., Sec. 36
- Civil township: Central
- Land area: 3.1 square miles
- Water area: 0.01 square miles (6.3 acres)
As the crow flies, St. Clair is...
| Year | Population | Percent change |
|---|
| 1900 | 189 | -- |
| 1910 | 397 | +110.1% |
| 1920 | 442 | +11.3% |
| 1930 | 1,135 | +156.8% |
| 1940 | 1,410 | +24.2% |
| 1950 | 1,779 | +26.2% |
| 1960 | 2,711 | +52.4% |
| 1970 | 2,978 | +9.8% |
| 1980 | 3,485 | +17.0% |
| 1990 | 3,917 | +12.4% |
| 2000 | 4,390 | +12.1% |