9 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Boulder | 1 | 105,995 | 108,418 | -0.38% | 3,941 | 27 | City |
| Longmont | 2 | 100,124 | 99,206 | 0.15% | 3,420 | 29 | City |
| Erie | 3 | 42,684 | 30,413 | 5.03% | 1,987 | 21 | Town |
| Lafayette | 4 | 30,679 | 30,403 | 0.15% | 3,323 | 9 | City |
| Louisville | 5 | 20,614 | 21,205 | -0.47% | 2,586 | 8 | City |
| Lyons | 6 | 2,130 | 2,205 | -0.56% | 1,578 | 1 | Town |
| Nederland | 7 | 1,495 | 1,470 | 0.27% | 1,010 | 1 | Town |
| Jamestown | 8 | 241 | 256 | -0.82% | 420 | 1 | Town |
| Ward | 9 | 124 | 132 | -0.8% | 231 | 1 | Town |
Boulder is the largest city in Boulder County, with a population of 105,995.
Ward is the smallest city in Boulder County, with a population of 124.
There are 9 cities in Boulder County, Colorado.