11 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| El Dorado | 1 | 16,319 | 17,694 | -1.66% | 1,007 | 16.20 | City |
| Smackover | 2 | 1,550 | 1,627 | -0.96% | 356 | 4.35 | City |
| Norphlet | 3 | 704 | 761 | -1.54% | 344 | 2.04 | City |
| Junction City | 4 | 468 | 498 | -1.27% | 412 | 1.14 | City |
| Huttig | 5 | 417 | 451 | -1.65% | 140 | 2.98 | City |
| Calion | 6 | 396 | 430 | -1.74% | 369 | 1.07 | City |
| Strong | 7 | 390 | 415 | -1.27% | 244 | 1.60 | City |
| Mount Holly | 8 | 230 | 102 | 6.98% | 82 | 2.79 | CDP |
| Lawson | 9 | 149 | 80 | 7.19% | 45 | 3.29 | CDP |
| Urbana | 10 | 149 | 168 | 7.19% | 80 | 1.86 | CDP |
| Felsenthal | 11 | 85 | 85 | 0% | 58 | 1.48 | Town |
El Dorado is the largest city in Union County, with a population of 16,319.
Felsenthal is the smallest city in Union County.
There are 11 cities in Union County, Arkansas.