20 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Big Sioux | 1 | 5,403 | 4,719 | 2.45% | 470 | 11 | Township |
| North Sioux City | 2 | 3,066 | 3,045 | 0.13% | 919 | 3 | City |
| Beresford | 3 | 2,041 | 2,179 | -1.11% | 1,144 | 2 | City |
| Elk Point | 4 | 2,039 | 2,144 | -0.83% | 1,316 | 2 | City |
| Jefferson | 5 | 1,267 | 1,110 | 2.34% | 33 | 39 | Township |
| Alcester | 6 | 748 | 820 | -1.58% | 727 | 1 | City |
| Jefferson | 7 | 437 | 473 | -1.35% | 857 | 1 | City |
| Civil Bend | 8 | 362 | 316 | 2.55% | 12 | 30 | Township |
| Alcester | 9 | 323 | 283 | 2.54% | 9 | 35 | Township |
| Elk Point | 10 | 299 | 260 | 2.75% | 5 | 55 | Township |
| Big Springs | 11 | 285 | 257 | 1.42% | 8 | 37 | Township |
| Virginia | 12 | 285 | 251 | 2.52% | 7 | 39 | Township |
| Brule | 13 | 283 | 246 | 2.91% | 8 | 36 | Township |
| Prairie | 14 | 264 | 235 | 1.93% | 8 | 35 | Township |
| Sioux Valley | 15 | 259 | 226 | 2.78% | 6 | 42 | Township |
| Emmet | 16 | 253 | 224 | 2.02% | 7 | 36 | Township |
| Spink | 17 | 244 | 213 | 2.52% | 7 | 36 | Township |
| Jefferson | 18 | 183 | 168 | 2.23% | 5 | 36 | Township |
| Jefferson | 19 | 93 | 91 | -1.06% | 3 | 35 | Township |
| Jefferson | 20 | 48 | 51 | -2.04% | 1 | 48 | Township |
Big Sioux is the largest city in Union County, with a population of 5,403.
Jefferson is the smallest city in Union County, with a population of 48.
There are 20 cities in Union County, South Dakota.