22 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Big Sioux | 1 | 5,203 | 4,721 | 1.82% | 453 | 11 | Township |
| Dakota Dunes | 2 | 4,996 | 3,231 | 7.14% | 2,023 | 2 | CDP |
| North Sioux City | 3 | 3,050 | 3,044 | 0.03% | 1,138 | 3 | City |
| Beresford | 4 | 2,053 | 2,178 | -1.2% | 1,151 | 2 | City |
| Elk Point | 5 | 2,031 | 2,140 | -1.07% | 1,311 | 2 | City |
| Jefferson | 6 | 1,227 | 1,110 | 2% | 32 | 39 | Township |
| Alcester | 7 | 751 | 825 | -1.96% | 1,023 | 1 | City |
| Jefferson | 8 | 431 | 473 | -1.82% | 845 | 1 | City |
| Civil Bend | 9 | 346 | 315 | 1.76% | 12 | 30 | Township |
| Alcester | 10 | 314 | 284 | 1.95% | 9 | 35 | Township |
| Big Springs | 11 | 286 | 259 | 2.14% | 8 | 37 | Township |
| Elk Point | 12 | 282 | 259 | 1.44% | 5 | 55 | Township |
| Virginia | 13 | 273 | 252 | 1.49% | 7 | 39 | Township |
| Brule | 14 | 265 | 246 | 1.15% | 7 | 36 | Township |
| Prairie | 15 | 259 | 235 | 1.97% | 7 | 35 | Township |
| Sioux Valley | 16 | 248 | 226 | 2.06% | 6 | 42 | Township |
| Emmet | 17 | 246 | 225 | 1.65% | 7 | 36 | Township |
| Spink | 18 | 234 | 213 | 1.74% | 6 | 36 | Township |
| Richland | 19 | 210 | 154 | 7.14% | 37 | 6 | CDP |
| Jefferson | 20 | 177 | 167 | 1.72% | 5 | 36 | Township |
| Jefferson | 21 | 102 | 91 | 3.03% | 3 | 35 | Township |
| Jefferson | 22 | 53 | 51 | 1.92% | 1 | 48 | Township |
Big Sioux is the largest city in Union County, with a population of 5,203.
Jefferson is the smallest city in Union County, with a population of 53.
There are 22 cities in Union County, South Dakota.