| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Ottawa | 1 | 18,663 | 18,803 | -0.11% | 1,253 | 15 | City |
| Streator | 2 | 12,133 | 12,462 | -0.47% | 1,658 | 7 | City |
| Peru | 3 | 9,746 | 9,920 | -0.35% | 933 | 10 | City |
| LaSalle | 4 | 9,351 | 9,564 | -0.39% | 695 | 13 | City |
| Mendota | 5 | 6,863 | 7,039 | -0.45% | 1,339 | 5 | City |
| Marseilles | 6 | 4,793 | 4,874 | -0.33% | 508 | 9 | City |
| Oglesby | 7 | 3,600 | 3,695 | -0.5% | 721 | 5 | City |
| Seneca | 8 | 2,301 | 2,347 | -0.39% | 386 | 6 | Village |
| Sheridan | 9 | 1,998 | 2,411 | -4.58% | 942 | 2 | Village |
| Earlville | 10 | 1,585 | 1,612 | -0.31% | 1,320 | 1 | City |
| North Utica | 11 | 1,337 | 1,322 | 0.22% | 387 | 3 | Village |
| Leland | 12 | 923 | 952 | -0.54% | 1,728 | 1 | Village |
| Tonica | 13 | 733 | 755 | -0.54% | 538 | 1 | Village |
| Grand Ridge | 14 | 505 | 516 | -0.59% | 1,093 | 0 | Village |
| Lostant | 15 | 427 | 433 | -0.23% | 378 | 1 | Village |
| Naplate | 16 | 413 | 419 | -0.24% | 2,534 | 0 | Village |
| Ransom | 17 | 309 | 310 | 0% | 309 | 1 | Village |
| Cedar Point | 18 | 257 | 265 | -0.77% | 252 | 1 | Village |
| Rutland | 19 | 253 | 256 | -0.39% | 365 | 1 | Village |
| Troy Grove | 20 | 225 | 226 | 0% | 328 | 1 | Village |
| Kangley | 21 | 223 | 230 | -0.89% | 854 | 0 | Village |
| Dana | 22 | 161 | 165 | -0.62% | 732 | 0 | Village |
| Leonore | 23 | 118 | 121 | -0.84% | 1,326 | 0 | Village |
Ottawa is the largest city in LaSalle County, with a population of 18,663.
Leonore is the smallest city in LaSalle County, with a population of 118.
There are 23 cities in LaSalle County, Illinois.