18 cities
| City | County | 2026 Pop. ↓ |
|---|---|---|
| Joliet | Will County | 152,579 |
| Jacksonville | Morgan County | 17,579 |
| Justice | Cook County | 12,118 |
| Jerseyville | Jersey County | 8,363 |
| Johnsburg | Lake County | 6,458 |
| Johnston City | Williamson County | 3,274 |
| Jonesboro | Union County | 1,630 |
| Jerome | Sangamon County | 1,626 |
| Junction City | Clinton County | 519 |
| Joy | Mercer County | 354 |
| Jeffersonville | Wayne County | 347 |
| Joppa | Massac County | 324 |
| Jewett | Cumberland County | 192 |
| Jeisyville | Christian County | 98 |
| Johnsonville | Wayne County | 70 |
| Joslin | Rock Island County | 54 |
| Junction | Gallatin County | 52 |
| Janesville | Cumberland County | 17 |