| | St. Onge, SD | Belle Fourche, SD | United States |
| Population | 13 | 5,561 | 329,725,481 |
| Female Population | 53.9% | 50.3% | 50.5% |
| Male Population | 46.2% | 49.7% | 49.5% |
| Median Age | 0 | 38.7 | 38.4 |
| space3 | | | |
| Population - 2010 | 231 | 5,614 | 309,327,143 |
| Population - 2000 | 85 | 4,565 | 285,036,114 |
| Population - 1990 | 72 | 4,335 | 251,960,433 |
| Pop. 2000 to Now | -52.9% | 25.8% | 15.6% |
| Pop. 1990 to Now | -44.4% | 32.5% | 30.8% |
| space4 | | | |
| Population Density | 9.7 | 628.5 | 93.4 |
| Land Area | 1.3 | 8.8 | 3,531,905.4 |
| Water Area | 0.0 | 0.1 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | St. Onge, SD | Belle Fourche, SD | United States |
| Age 0 to 4 | 0.0% | 4.6% | 5.9% |
| Age 5 to 9 | 0.0% | 7.2% | 6.1% |
| Age 10 to 14 | 0.0% | 5.5% | 6.6% |
| Age 15 to 17 | 0.0% | 5.4% | 3.9% |
| Age 18 to 20 | 0.0% | 4.6% | 4.0% |
| Age 21 to 24 | 0.0% | 4.7% | 5.2% |
| Age 25 to 34 | 0.0% | 12.1% | 13.8% |
| Age 35 to 44 | 0.0% | 12.2% | 12.9% |
| Age 45 to 54 | 0.0% | 13.2% | 12.6% |
| Age 55 to 59 | 0.0% | 5.4% | 6.7% |
| Age 60 to 64 | 0.0% | 9.4% | 6.3% |
| Age 65 to 74 | 0.0% | 7.8% | 9.6% |
| Age 75 to 84 | 100.0% | 5.7% | 4.5% |
| Age 85 and over | 0.0% | 2.5% | 1.9% |
| space1 | | | |
| Race | | | |
| | St. Onge, SD | Belle Fourche, SD | United States |
| White | 0.0% | 87.0% | 59.4% |
| Black | 0.0% | 0.0% | 12.2% |
| Asian | 0.0% | 0.4% | 5.6% |
| Native American | 0.0% | 1.4% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.5% | 0.2% |
| Other | 0.0% | 0.6% | 0.4% |
| Two or More Races | 0.0% | 3.1% | 3.2% |
| Hispanic | 100.0% | 7.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | St. Onge, SD | Belle Fourche, SD | United States |
| Married Population | 100.0% | 49.0% | 49.8% |
| Currently Married | 100.0% | 47.5% | 48.0% |
| Married but Separated | 0.0% | 1.4% | 1.8% |
| Single Population | 0.0% | 51.0% | 50.2% |
| Never Married | 0.0% | 29.0% | 33.8% |
| Divorced | 0.0% | 17.0% | 10.8% |
| Widowed | 0.0% | 5.0% | 5.6% |
| space5 | | | |
| Household Size | 2.17 | 2.35 | 2.60 |
| Households | 6 | 2,327 | 124,010,992 |
| Family Households | 6 | 1,302 | 80,755,759 |
| Married couple, w/children | 0.0% | 41.8% | 30.5% |
| Married couple, no children | 100.0% | 41.5% | 42.9% |
| Non Family Households | 0 | 1,025 | 43,255,233 |
| Single householder, w/children | 0.0% | 9.8% | 15.8% |
| Single householder, no children | 0.0% | 6.9% | 10.8% |