21 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Middletown | 1 | 49,026 | 46,976 | 0.84% | 1,195 | 41 | City |
| Cromwell | 2 | 14,402 | 14,214 | 0.1% | 1,157 | 12 | Township |
| Clinton | 3 | 13,485 | 13,173 | 0.13% | 832 | 16 | Township |
| East Hampton | 4 | 13,192 | 12,697 | 0.63% | 370 | 36 | Township |
| Old Saybrook | 5 | 10,756 | 10,468 | 0.63% | 714 | 15 | Township |
| Portland | 6 | 9,503 | 9,361 | 0.26% | 407 | 23 | Township |
| East Haddam | 7 | 9,106 | 8,858 | 0.5% | 168 | 54 | Township |
| Haddam | 8 | 8,815 | 8,437 | 0.1% | 201 | 44 | Township |
| Old Lyme | 9 | 7,733 | 7,604 | 0.16% | 336 | 23 | Township |
| Durham | 10 | 7,237 | 7,141 | 0.1% | 306 | 24 | Township |
| Westbrook | 11 | 6,934 | 6,761 | 0.23% | 440 | 16 | Township |
| Essex | 12 | 6,860 | 6,728 | 0.19% | 659 | 10 | Township |
| Portland | 13 | 6,556 | 5,937 | 0.71% | 1,094 | 6 | CDP |
| Killingworth | 14 | 6,298 | 6,162 | 0.22% | 178 | 35 | Township |
| Deep River | 15 | 4,498 | 4,405 | 0.33% | 333 | 14 | Township |
| Middlefield | 16 | 4,281 | 4,208 | 0.16% | 338 | 13 | Township |
| Chester | 17 | 3,752 | 3,739 | 0.11% | 234 | 16 | Township |
| Durham | 18 | 3,603 | 3,542 | 1.26% | 437 | 8 | CDP |
| Clinton | 19 | 3,049 | 3,309 | -0.68% | 1,274 | 2 | CDP |
| East Hampton | 20 | 3,007 | 3,526 | 0.7% | 742 | 4 | CDP |
| Lyme | 21 | 2,425 | 2,349 | 0.17% | 76 | 32 | Township |
Middletown is the largest city in Lower CT River Valley Planning Region, with a population of 49,026.
Lyme is the smallest city in Lower CT River Valley Planning Region, with a population of 2,425.
There are 21 cities in Lower CT River Valley Planning Region, Connecticut.