5 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| North Sioux City | 1 | 3,066 | 3,045 | 0.13% | 919 | 3 | City |
| Beresford | 2 | 2,041 | 2,179 | -1.11% | 1,144 | 2 | City |
| Elk Point | 3 | 2,039 | 2,144 | -0.83% | 1,316 | 2 | City |
| Alcester | 4 | 748 | 820 | -1.58% | 727 | 1 | City |
| Jefferson | 5 | 437 | 473 | -1.35% | 857 | 1 | City |
North Sioux City is the largest city in Union County, with a population of 3,066.
Jefferson is the smallest city in Union County, with a population of 437.
There are 5 cities in Union County, South Dakota.