14 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| East Buffalo | 1 | 7,439 | 7,395 | -0.17% | 488 | 15 | Township |
| Lewisburg | 2 | 5,301 | 5,156 | 0.45% | 5,448 | 1 | Borough |
| White Deer | 3 | 4,332 | 4,404 | -0.48% | 94 | 46 | Township |
| Kelly | 4 | 4,253 | 4,001 | 5.32% | 248 | 17 | Township |
| Buffalo | 5 | 3,553 | 3,588 | -0.25% | 118 | 30 | Township |
| Mifflinburg | 6 | 3,464 | 3,482 | -0.09% | 1,920 | 2 | Borough |
| Gregg | 7 | 3,395 | 4,092 | -5.25% | 228 | 15 | Township |
| Winfield | 8 | 3,291 | 3,362 | -0.06% | 138 | 24 | Township |
| West Buffalo | 9 | 2,829 | 2,883 | -0.46% | 74 | 38 | Township |
| Hartley | 10 | 1,705 | 1,749 | -0.58% | 21 | 80 | Township |
| Union | 11 | 1,603 | 1,630 | -0.31% | 144 | 11 | Township |
| Lewis | 12 | 1,471 | 1,482 | 0.07% | 38 | 39 | Township |
| New Berlin | 13 | 770 | 794 | -0.52% | 1,985 | 0 | Borough |
| Hartleton | 14 | 246 | 251 | -0.4% | 271 | 1 | Borough |
East Buffalo is the largest city in Union County, with a population of 7,439.
Hartleton is the smallest city in Union County, with a population of 246.
There are 14 cities in Union County, Pennsylvania.