31 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Boulder | 1 | 104,414 | 108,123 | -0.71% | 3,882 | 26.89 | City |
| Longmont | 2 | 98,258 | 99,188 | -0.19% | 3,384 | 29.04 | City |
| Erie | 3 | 38,503 | 30,417 | 4.38% | 1,857 | 20.74 | Town |
| Lafayette | 4 | 30,443 | 30,433 | 0.01% | 3,298 | 9.23 | City |
| Louisville | 5 | 19,867 | 21,174 | -1.3% | 2,492 | 7.97 | City |
| Gunbarrel | 6 | 10,319 | 10,494 | 0.68% | 1,652 | 6.25 | CDP |
| Niwot | 7 | 4,112 | 4,085 | 0.17% | 1,029 | 4 | CDP |
| Lyons | 8 | 2,113 | 2,207 | -0.89% | 1,565 | 1.35 | Town |
| Nederland | 9 | 1,502 | 1,467 | 0.47% | 1,015 | 1.48 | Town |
| Lazy Acres | 10 | 1,038 | 1,071 | 0.78% | 196 | 5.29 | CDP |
| Paragon Estates | 11 | 839 | 785 | -0.71% | 501 | 1.68 | CDP |
| Allenspark | 12 | 646 | 464 | 1.25% | 54 | 11.94 | CDP |
| Sugarloaf | 13 | 561 | 262 | 3.7% | 258 | 2.18 | CDP |
| Altona | 14 | 538 | 479 | 0.37% | 319 | 1.69 | CDP |
| St. Ann Highlands | 15 | 493 | 486 | 2.92% | 354 | 1.39 | CDP |
| Tall Timber | 16 | 460 | 295 | 3.6% | 801 | 0.57 | CDP |
| Eldora | 17 | 441 | 352 | 4.75% | 106 | 4.16 | CDP |
| Pine Brook Hill | 18 | 406 | 776 | -8.56% | 134 | 3.03 | CDP |
| Crisman | 19 | 284 | 51 | 2.53% | 195 | 1.46 | CDP |
| Gold Hill | 20 | 254 | 195 | 0.79% | 122 | 2.08 | CDP |
| Jamestown | 21 | 238 | 255 | -1.24% | 415 | 0.57 | Town |
| Eldorado Springs | 22 | 209 | 325 | -10.68% | 81 | 2.58 | CDP |
| Sunshine | 23 | 208 | 145 | -0.48% | 120 | 1.73 | CDP |
| Mountain Meadows | 24 | 155 | 174 | -4.91% | 90 | 1.72 | CDP |
| Seven Hills | 25 | 136 | 127 | 0.74% | 278 | 0.49 | CDP |
| Bonanza Mountain Estates | 26 | 134 | 174 | 0% | 798 | 0.17 | CDP |
| Ward | 27 | 123 | 130 | -0.81% | 229 | 0.54 | Town |
| Bark Ranch | 28 | 108 | 107 | -6.09% | 125 | 0.86 | CDP |
| Valmont | 29 | 101 | 98 | 7.45% | 391 | 0.26 | CDP |
| Glendale | 30 | 85 | 109 | 7.59% | 68 | 1.25 | CDP |
| Hidden Lake | 31 | 36 | 30 | 5.88% | 59 | 0.61 | CDP |
Boulder is the largest city in Boulder County, with a population of 104,414.
Hidden Lake is the smallest city in Boulder County.
There are 31 cities in Boulder County, Colorado.