35 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| DeKalb township | 1 | 42,613 | 42,638 | -0.02% | 1,215 | 35.07 | Township |
| DeKalb | 2 | 40,205 | 40,219 | -0.01% | 2,320 | 17.33 | City |
| Sycamore | 3 | 18,752 | 18,576 | 0.19% | 1,853 | 10.12 | City |
| Sycamore township | 4 | 14,707 | 14,690 | -0.01% | 429 | 34.28 | Township |
| Pingree Grove | 5 | 11,629 | 10,399 | 2.16% | 2,814 | 4.13 | Village |
| Cortland township | 6 | 11,426 | 11,188 | 0.43% | 326 | 35.03 | Township |
| Sandwich township | 7 | 7,604 | 7,513 | 0.68% | 498 | 15.26 | Township |
| Sandwich | 8 | 7,212 | 7,212 | 0% | 1,523 | 4.74 | City |
| Genoa township | 9 | 5,756 | 5,538 | 1.05% | 160 | 36.05 | Township |
| Lake Holiday | 10 | 5,666 | 5,365 | 1.07% | 1,617 | 3.50 | CDP |
| Genoa | 11 | 5,523 | 5,307 | 0.78% | 2,128 | 2.60 | City |
| Cortland | 12 | 4,548 | 4,397 | 0.66% | 1,266 | 3.59 | Town |
| Kingston township | 13 | 3,461 | 3,471 | 0.03% | 97 | 35.74 | Township |
| Squaw Grove township | 14 | 2,647 | 2,724 | -0.71% | 76 | 34.99 | Township |
| Hinckley | 15 | 1,953 | 1,997 | -0.46% | 2,279 | 0.86 | Village |
| Somonauk township | 16 | 1,941 | 1,996 | -0.67% | 101 | 19.27 | Township |
| Somonauk | 17 | 1,743 | 1,785 | -0.46% | 697 | 2.50 | Village |
| Kirkland | 18 | 1,598 | 1,648 | -0.62% | 1,287 | 1.24 | Village |
| Malta township | 19 | 1,509 | 1,530 | -0.2% | 43 | 34.96 | Township |
| Waterman | 20 | 1,413 | 1,433 | -0.28% | 943 | 1.50 | Village |
| Shabbona township | 21 | 1,311 | 1,350 | -0.61% | 38 | 34.65 | Township |
| Hillcrest | 22 | 1,207 | 1,221 | -0.25% | 340 | 3.55 | Village |
| Malta | 23 | 1,125 | 1,142 | -0.27% | 1,829 | 0.61 | Village |
| Kingston | 24 | 1,077 | 1,106 | -0.55% | 1,095 | 0.98 | Village |
| Lily Lake | 25 | 1,045 | 1,035 | 0.19% | 385 | 2.72 | Village |
| Afton township | 26 | 893 | 906 | -0.33% | 25 | 35.11 | Township |
| Mayfield township | 27 | 887 | 909 | -0.56% | 25 | 34.93 | Township |
| Shabbona | 28 | 840 | 865 | -0.59% | 655 | 1.28 | Village |
| Kings | 29 | 434 | 348 | 7.16% | 2,837 | 0.15 | CDP |
| Pierce township | 30 | 426 | 438 | -0.7% | 12 | 34.98 | Township |
| South Grove township | 31 | 406 | 421 | -0.98% | 12 | 34.41 | Township |
| Victor township | 32 | 284 | 290 | -0.35% | 9 | 32.75 | Township |
| Virgil | 33 | 279 | 289 | -0.71% | 128 | 2.18 | Village |
| Kingston | 34 | 148 | 107 | 7.25% | 695 | 0.21 | CDP |
| Harding | 35 | 138 | 153 | 7.81% | 812 | 0.17 | CDP |
DeKalb township is the largest city in DeKalb County, with a population of 42,613.
Harding is the smallest city in DeKalb County.
There are 35 cities in DeKalb County, Illinois.