| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| East Buffalo | 1 | 8,079 | 7,396 | -0.06% | 530 | 15 | Township |
| Lewisburg | 2 | 5,246 | 5,155 | 0.38% | 5,392 | 1 | Borough |
| White Deer | 3 | 4,353 | 4,418 | -0.23% | 94 | 46 | Township |
| Buffalo | 4 | 3,540 | 3,592 | -0.17% | 117 | 30 | Township |
| Kelly | 5 | 3,536 | 3,990 | -3.57% | 206 | 17 | Township |
| Gregg | 6 | 3,450 | 4,093 | -4.01% | 232 | 15 | Township |
| Mifflinburg | 7 | 3,428 | 3,475 | -0.23% | 1,900 | 2 | Borough |
| Winfield | 8 | 3,277 | 3,357 | -0.3% | 138 | 24 | Township |
| West Buffalo | 9 | 2,844 | 2,888 | -0.28% | 74 | 38 | Township |
| Hartley | 10 | 1,717 | 1,747 | 0% | 22 | 80 | Township |
| Union | 11 | 1,616 | 1,632 | 0.25% | 145 | 11 | Township |
| Lewis | 12 | 1,458 | 1,480 | -0.21% | 38 | 39 | Township |
| New Berlin | 13 | 766 | 794 | -0.65% | 1,974 | 0 | Borough |
| Hartleton | 14 | 244 | 250 | -0.41% | 269 | 1 | Borough |
East Buffalo is the largest city in Union County, with a population of 8,079.
Hartleton is the smallest city in Union County, with a population of 244.
There are 14 cities in Union County, Pennsylvania.