18 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Joliet | Will County | 152,208 |
| Jacksonville | Morgan County | 17,690 |
| Justice | Cook County | 12,192 |
| Jerseyville | Jersey County | 8,356 |
| Johnsburg | Lake County | 6,443 |
| Johnston City | Williamson County | 3,291 |
| Jonesboro | Union County | 1,642 |
| Jerome | Sangamon County | 1,636 |
| Junction City | Clinton County | 521 |
| Joy | Mercer County | 356 |
| Jeffersonville | Wayne County | 348 |
| Joppa | Massac County | 328 |
| Jewett | Cumberland County | 193 |
| Jeisyville | Christian County | 99 |
| Joslin | Rock Island County | 74 |
| Johnsonville | Wayne County | 70 |
| Janesville | Cumberland County | 55 |
| Junction | Gallatin County | 53 |