14 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| East Buffalo township | 1 | 7,380 | 7,392 | -0.45% | 484 | 15.25 | Township |
| Lewisburg | 2 | 5,323 | 5,153 | 0.64% | 5,471 | 0.97 | Borough |
| White Deer township | 3 | 4,299 | 4,396 | -0.69% | 93 | 46.16 | Township |
| Buffalo township | 4 | 3,498 | 3,593 | -0.91% | 116 | 30.13 | Township |
| Gregg township | 5 | 3,460 | 4,091 | -6.64% | 232 | 14.88 | Township |
| Mifflinburg | 6 | 3,419 | 3,473 | -0.32% | 1,895 | 1.80 | Borough |
| Winfield township | 7 | 3,286 | 3,368 | -0.21% | 138 | 23.82 | Township |
| Kelly township | 8 | 3,236 | 4,014 | -5.38% | 188 | 17.17 | Township |
| West Buffalo township | 9 | 2,794 | 2,883 | -0.92% | 73 | 38.26 | Township |
| Hartley township | 10 | 1,703 | 1,746 | -0.64% | 21 | 79.60 | Township |
| Union township | 11 | 1,586 | 1,630 | -0.81% | 142 | 11.15 | Township |
| Lewis township | 12 | 1,446 | 1,478 | -0.55% | 37 | 38.72 | Township |
| New Berlin | 13 | 777 | 802 | -0.64% | 2,003 | 0.39 | Borough |
| Hartleton | 14 | 246 | 250 | -0.4% | 271 | 0.91 | Borough |
East Buffalo township is the largest city in Union County, with a population of 7,380.
Hartleton is the smallest city in Union County.
There are 14 cities in Union County, Pennsylvania.