| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| North Sioux City | 1 | 3,039 | 3,051 | 0.1% | 911 | 3 | City |
| Beresford | 2 | 2,082 | 2,178 | -1.09% | 1,167 | 2 | City |
| Elk Point | 3 | 2,061 | 2,144 | -0.91% | 1,331 | 2 | City |
| Alcester | 4 | 763 | 823 | -1.68% | 741 | 1 | City |
| Jefferson | 5 | 441 | 472 | -1.56% | 865 | 1 | City |
North Sioux City is the largest city in Union County, with a population of 3,039.
Jefferson is the smallest city in Union County, with a population of 441.
There are 5 cities in Union County, South Dakota.