9 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Clayton | 1 | 2,536 | 2,655 | -0.9% | 325 | 8 | Town |
| Clayton | 2 | 2,536 | 2,649 | -0.9% | 325 | 8 | Town |
| Des Moines | 3 | 109 | 117 | -1.8% | 125 | 1 | Village |
| Des Moines | 4 | 109 | 116 | -1.8% | 125 | 1 | Village |
| Capulin | 5 | 93 | 19 | 8.14% | 63 | 1 | CDP |
| Folsom | 6 | 44 | 47 | -2.22% | 68 | 1 | Village |
| Folsom | 7 | 44 | 47 | -2.22% | 68 | 1 | Village |
| Grenville | 8 | 24 | 24 | 0% | 63 | 0 | Village |
| Grenville | 9 | 24 | 24 | 0% | 63 | 0 | Village |
Clayton is the largest city in Union County, with a population of 2,536.
Grenville is the smallest city in Union County, with a population of 24.
There are 9 cities in Union County, New Mexico.