31 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% | 3,443 | 29 | City |
| Erie | 3 | 40,639 | 30,413 | 5.3% | 1,960 | 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 |
| Gunbarrel | 6 | 10,319 | 10,494 | 0.68% | 1,652 | 6 | CDP |
| Niwot | 7 | 4,112 | 4,085 | 0.17% | 1,029 | 4 | CDP |
| Lyons | 8 | 2,142 | 2,205 | -0.6% | 1,587 | 1 | Town |
| Nederland | 9 | 1,491 | 1,470 | 0.27% | 1,007 | 1 | Town |
| Lazy Acres | 10 | 1,038 | 1,071 | 0.78% | 196 | 5 | CDP |
| Paragon Estates | 11 | 839 | 785 | -0.71% | 501 | 2 | CDP |
| Allenspark | 12 | 646 | 464 | 1.25% | 54 | 12 | CDP |
| Sugarloaf | 13 | 561 | 262 | 3.7% | 258 | 2 | CDP |
| Altona | 14 | 538 | 479 | 0.37% | 319 | 2 | CDP |
| St. Ann Highlands | 15 | 493 | 486 | 2.92% | 354 | 1 | CDP |
| Tall Timber | 16 | 460 | 295 | 3.6% | 801 | 1 | CDP |
| Eldora | 17 | 441 | 352 | 4.75% | 106 | 4 | CDP |
| Pine Brook Hill | 18 | 406 | 776 | -8.56% | 134 | 3 | CDP |
| Crisman | 19 | 284 | 51 | 2.53% | 195 | 1 | CDP |
| Gold Hill | 20 | 254 | 195 | 0.79% | 122 | 2 | CDP |
| Jamestown | 21 | 243 | 256 | -1.22% | 423 | 1 | Town |
| Eldorado Springs | 22 | 209 | 325 | -10.68% | 81 | 3 | CDP |
| Sunshine | 23 | 208 | 145 | -0.48% | 120 | 2 | CDP |
| Mountain Meadows | 24 | 155 | 174 | -4.91% | 90 | 2 | CDP |
| Seven Hills | 25 | 136 | 127 | 0.74% | 278 | 0 | CDP |
| Bonanza Mountain Estates | 26 | 134 | 174 | 0% | 798 | 0 | CDP |
| Ward | 27 | 125 | 132 | -1.57% | 233 | 1 | Town |
| Bark Ranch | 28 | 108 | 107 | -6.09% | 125 | 1 | CDP |
| Valmont | 29 | 101 | 98 | 7.45% | 391 | 0 | CDP |
| Glendale | 30 | 85 | 109 | 7.59% | 68 | 1 | CDP |
| Hidden Lake | 31 | 36 | 30 | 5.88% | 59 | 1 | CDP |
Boulder is the largest city in Boulder County, with a population of 106,399.
Hidden Lake is the smallest city in Boulder County, with a population of 36.
There are 31 cities in Boulder County, Colorado.