21 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 |
| Linntown | 10 | 1,736 | 1,799 | 0.93% | 2,327 | 0.75 | CDP |
| Hartley township | 11 | 1,703 | 1,746 | -0.64% | 21 | 79.60 | Township |
| Union township | 12 | 1,586 | 1,630 | -0.81% | 142 | 11.15 | Township |
| Lewis township | 13 | 1,446 | 1,478 | -0.55% | 37 | 38.72 | Township |
| Winfield | 14 | 836 | 872 | -0.48% | 291 | 2.87 | CDP |
| New Columbia | 15 | 816 | 1,164 | -1.57% | 423 | 1.93 | CDP |
| New Berlin | 16 | 777 | 802 | -0.64% | 2,003 | 0.39 | Borough |
| West Milton | 17 | 596 | 1,152 | -3.25% | 718 | 0.83 | CDP |
| Laurelton | 18 | 402 | 336 | 3.08% | 705 | 0.57 | CDP |
| Allenwood | 19 | 312 | 195 | -0.32% | 531 | 0.59 | CDP |
| Hartleton | 20 | 246 | 250 | -0.4% | 271 | 0.91 | Borough |
| Vicksburg | 21 | 136 | 283 | -5.56% | 282 | 0.48 | CDP |
East Buffalo township is the largest city in Union County, with a population of 7,380.
Vicksburg is the smallest city in Union County.
There are 21 cities in Union County, Pennsylvania.