| City | County | 2026 Pop. ↓ |
|---|---|---|
| Joliet | Will County | 152,625 |
| Jacksonville | Morgan County | 17,644 |
| Justice | Cook County | 12,212 |
| Jerseyville | Jersey County | 8,414 |
| Johnsburg | Lake County | 6,458 |
| Johnston City | Williamson County | 3,295 |
| Jonesboro | Union County | 1,621 |
| Jerome | Sangamon County | 1,616 |
| Junction City | Clinton County | 514 |
| Joy | Mercer County | 354 |
| Jeffersonville | Wayne County | 347 |
| Joppa | Massac County | 326 |
| Jewett | Cumberland County | 185 |
| Jeisyville | Christian County | 98 |
| Johnsonville | Wayne County | 79 |
| Joslin | Rock Island County | 54 |
| Junction | Gallatin County | 51 |
| Janesville | Cumberland County | 17 |