9 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Boulder | 1 | 106,399 | 108,418 | -0.38% | 3,956 | 27 | City |
| Longmont | 2 | 99,971 | 99,206 | 0.15% | 3,414 | 29 | City |
| Erie | 3 | 40,639 | 30,413 | 5.3% | 1,892 | 21 | Town |
| Lafayette | 4 | 30,633 | 30,403 | 0.15% | 3,318 | 9 | City |
| Louisville | 5 | 20,712 | 21,205 | -0.48% | 2,598 | 8 | City |
| Lyons | 6 | 2,142 | 2,205 | -0.6% | 1,587 | 1 | Town |
| Nederland | 7 | 1,491 | 1,470 | 0.27% | 1,007 | 1 | Town |
| Jamestown | 8 | 243 | 256 | -1.22% | 423 | 1 | Town |
| Ward | 9 | 125 | 132 | -1.57% | 233 | 1 | Town |
Boulder is the largest city in Boulder County, with a population of 106,399.
Ward is the smallest city in Boulder County, with a population of 125.
There are 9 cities in Boulder County, Colorado.