| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| La Grande | 1 | 12,970 | 13,077 | -0.12% | 2,830 | 5 | City |
| Union | 2 | 2,109 | 2,152 | -0.42% | 847 | 2 | City |
| Elgin | 3 | 1,683 | 1,714 | -0.36% | 1,665 | 1 | City |
| Island City | 4 | 1,138 | 1,159 | -0.35% | 1,157 | 1 | City |
| Cove | 5 | 608 | 618 | -0.33% | 756 | 1 | City |
| North Powder | 6 | 499 | 504 | -0.2% | 795 | 1 | City |
| Imbler | 7 | 241 | 246 | -0.41% | 1,116 | 0 | City |
| Summerville | 8 | 118 | 119 | -0.84% | 447 | 0 | Town |
La Grande is the largest city in Union County, with a population of 12,970.
Summerville is the smallest city in Union County, with a population of 118.
There are 8 cities in Union County, Oregon.