18 cities
| City | County | 2025 Pop. ↓ |
|---|---|---|
| Joliet | Will County | 150,647 |
| Jacksonville | Morgan County | 17,003 |
| Justice | Cook County | 11,759 |
| Jerseyville | Jersey County | 8,223 |
| Johnsburg | Lake County | 6,404 |
| Johnston City | Williamson County | 3,260 |
| Jerome | Sangamon County | 1,616 |
| Jonesboro | Union County | 1,603 |
| Junction City | Clinton County | 515 |
| Joy | Mercer County | 360 |
| Jeffersonville | Wayne County | 338 |
| Joppa | Massac County | 323 |
| Jewett | Cumberland County | 195 |
| Jeisyville | Christian County | 98 |
| Joslin | Rock Island County | 74 |
| Johnsonville | Wayne County | 68 |
| Janesville | Cumberland County | 55 |
| Junction | Gallatin County | 51 |