31 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| DeKalb | 1 | 43,307 | 42,643 | 0.43% | 1,235 | 35 | Township |
| DeKalb | 2 | 40,624 | 40,224 | 0.17% | 2,279 | 18 | City |
| Sycamore | 3 | 19,114 | 18,675 | 0.38% | 1,888 | 10 | City |
| Sycamore | 4 | 15,099 | 14,789 | 0.55% | 440 | 34 | Township |
| Pingree Grove | 5 | 12,069 | 10,398 | 2.37% | 2,987 | 4 | Village |
| Cortland | 6 | 11,741 | 11,183 | 1.11% | 335 | 35 | Township |
| Sandwich | 7 | 7,734 | 7,515 | 0.91% | 507 | 15 | Township |
| Sandwich | 8 | 7,343 | 7,211 | 0.3% | 1,550 | 5 | City |
| Genoa | 9 | 5,812 | 5,539 | 0.92% | 161 | 36 | Township |
| Genoa | 10 | 5,617 | 5,307 | 0.93% | 2,164 | 3 | City |
| Cortland | 11 | 4,690 | 4,396 | 1.06% | 1,305 | 4 | Town |
| Kingston | 12 | 3,520 | 3,468 | 0.51% | 98 | 36 | Township |
| Squaw Grove | 13 | 2,706 | 2,725 | 0.15% | 77 | 35 | Township |
| Somonauk | 14 | 1,972 | 1,997 | -0.05% | 102 | 19 | Township |
| Hinckley | 15 | 1,966 | 1,996 | -0.25% | 2,294 | 1 | Village |
| Somonauk | 16 | 1,750 | 1,782 | -0.28% | 700 | 3 | Village |
| Kirkland | 17 | 1,608 | 1,646 | -0.37% | 1,295 | 1 | Village |
| Waterman | 18 | 1,599 | 1,430 | 1.78% | 1,067 | 1 | Village |
| Malta | 19 | 1,528 | 1,533 | 0.13% | 44 | 35 | Township |
| Shabbona | 20 | 1,337 | 1,351 | 0.15% | 39 | 35 | Township |
| Hillcrest | 21 | 1,198 | 1,222 | -0.33% | 338 | 4 | Village |
| Malta | 22 | 1,134 | 1,143 | -0.09% | 1,844 | 1 | Village |
| Kingston | 23 | 1,090 | 1,105 | -0.18% | 1,108 | 1 | Village |
| Lily Lake | 24 | 1,070 | 1,037 | 0.56% | 394 | 3 | Village |
| Afton | 25 | 910 | 905 | 0.33% | 26 | 35 | Township |
| Mayfield | 26 | 910 | 910 | 0.33% | 26 | 35 | Township |
| Shabbona | 27 | 845 | 866 | -0.35% | 659 | 1 | Village |
| Pierce | 28 | 428 | 438 | -0.47% | 12 | 35 | Township |
| South Grove | 29 | 415 | 421 | 0% | 12 | 34 | Township |
| Victor | 30 | 289 | 290 | 0.35% | 9 | 33 | Township |
| Virgil | 31 | 282 | 288 | -0.35% | 129 | 2 | Village |
DeKalb is the largest city in DeKalb County, with a population of 43,307.
Virgil is the smallest city in DeKalb County, with a population of 282.
There are 31 cities in DeKalb County, Illinois.