| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Boulder | 1 | 105,220 | 108,431 | -0.44% | 3,912 | 27 | City |
| Longmont | 2 | 100,211 | 99,192 | 0.1% | 3,423 | 29 | City |
| Erie | 3 | 42,967 | 30,422 | 5.04% | 2,000 | 21 | Town |
| Lafayette | 4 | 30,150 | 30,390 | 0.11% | 3,266 | 9 | City |
| Louisville | 5 | 20,468 | 21,205 | -0.55% | 2,568 | 8 | City |
| Lyons | 6 | 2,105 | 2,204 | -0.66% | 1,559 | 1 | Town |
| Nederland | 7 | 1,466 | 1,468 | 0.21% | 991 | 1 | Town |
| Jamestown | 8 | 242 | 259 | -1.22% | 422 | 1 | Town |
| Ward | 9 | 122 | 132 | -1.61% | 228 | 1 | Town |
Boulder is the largest city in Boulder County, with a population of 105,220.
Ward is the smallest city in Boulder County, with a population of 122.
There are 9 cities in Boulder County, Colorado.