5 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Clayton | 1 | 2,536 | 2,648 | -0.86% | 325 | 7.81 | Town |
| Des Moines | 2 | 110 | 115 | -0.9% | 126 | 0.87 | Village |
| Capulin | 3 | 93 | 19 | 8.14% | 63 | 1.48 | CDP |
| Folsom | 4 | 46 | 48 | 0% | 71 | 0.65 | Village |
| Grenville | 5 | 25 | 24 | 0% | 66 | 0.38 | Village |
Clayton is the largest city in Union County, with a population of 2,536.
Grenville is the smallest city in Union County.
There are 5 cities in Union County, New Mexico.