| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Claremont | 1 | 12,998 | 12,952 | -0.26% | 301 | 43 | City |
| Newport | 2 | 6,432 | 6,301 | 0.23% | 148 | 44 | Town |
| Charlestown | 3 | 4,926 | 4,806 | 0.04% | 138 | 36 | Town |
| Grantham | 4 | 3,491 | 3,407 | 0.26% | 128 | 27 | Town |
| Sunapee | 5 | 3,439 | 3,349 | -0.09% | 163 | 21 | Town |
| Plainfield | 6 | 2,570 | 2,461 | 0.39% | 49 | 52 | Town |
| Cornish | 7 | 1,647 | 1,614 | 0.06% | 39 | 42 | Town |
| Unity | 8 | 1,525 | 1,516 | -0.2% | 41 | 37 | Town |
| Springfield | 9 | 1,304 | 1,245 | -0.15% | 30 | 44 | Town |
| Washington | 10 | 1,233 | 1,193 | 0.98% | 27 | 46 | Town |
| Lempster | 11 | 1,169 | 1,113 | 0.6% | 36 | 32 | Town |
| Acworth | 12 | 905 | 856 | 1% | 23 | 39 | Town |
| Croydon | 13 | 891 | 812 | 1.71% | 24 | 37 | Town |
| Goshen | 14 | 810 | 798 | 0.12% | 36 | 22 | Town |
| Langdon | 15 | 658 | 650 | -0.3% | 40 | 16 | Town |
Claremont is the largest city in Sullivan County, with a population of 12,998.
Langdon is the smallest city in Sullivan County, with a population of 658.
There are 15 cities in Sullivan County, New Hampshire.