20 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Big Sioux | 1 | 5,274 | 4,719 | 2.51% | 459 | 11 | Township |
| North Sioux City | 2 | 3,062 | 3,045 | 0.1% | 918 | 3 | City |
| Beresford | 3 | 2,064 | 2,179 | -1.1% | 1,157 | 2 | City |
| Elk Point | 4 | 2,056 | 2,144 | -0.87% | 1,327 | 2 | City |
| Jefferson | 5 | 1,238 | 1,110 | 2.4% | 32 | 39 | Township |
| Alcester | 6 | 760 | 820 | -1.55% | 739 | 1 | City |
| Jefferson | 7 | 443 | 473 | -1.34% | 869 | 1 | City |
| Civil Bend | 8 | 353 | 316 | 2.62% | 12 | 30 | Township |
| Alcester | 9 | 315 | 283 | 2.61% | 9 | 35 | Township |
| Elk Point | 10 | 291 | 260 | 2.83% | 5 | 55 | Township |
| Big Springs | 11 | 281 | 257 | 1.44% | 8 | 37 | Township |
| Virginia | 12 | 278 | 251 | 2.58% | 7 | 39 | Township |
| Brule | 13 | 275 | 246 | 3% | 8 | 36 | Township |
| Prairie | 14 | 259 | 235 | 1.97% | 7 | 35 | Township |
| Sioux Valley | 15 | 252 | 226 | 2.86% | 6 | 42 | Township |
| Emmet | 16 | 248 | 224 | 2.06% | 7 | 36 | Township |
| Spink | 17 | 238 | 213 | 2.59% | 7 | 36 | Township |
| Jefferson | 18 | 179 | 168 | 2.29% | 5 | 36 | Township |
| Jefferson | 19 | 94 | 91 | -1.05% | 3 | 35 | Township |
| Jefferson | 20 | 49 | 51 | -2% | 1 | 48 | Township |
Big Sioux is the largest city in Union County, with a population of 5,274.
Jefferson is the smallest city in Union County, with a population of 49.
There are 20 cities in Union County, South Dakota.