| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Elizabeth | 1 | 141,795 | 136,609 | 0.08% | 11,510 | 12 | City |
| Union | 2 | 63,732 | 59,768 | 0.09% | 7,041 | 9 | Township |
| Plainfield | 3 | 57,385 | 54,308 | 0.43% | 9,630 | 6 | City |
| Linden | 4 | 45,770 | 43,521 | 0.04% | 4,282 | 11 | City |
| Westfield | 5 | 32,892 | 30,890 | 0.91% | 4,892 | 7 | Town |
| Rahway | 6 | 31,007 | 29,678 | -0.01% | 7,959 | 4 | City |
| Scotch Plains | 7 | 25,450 | 24,838 | 0.05% | 2,821 | 9 | Township |
| Cranford | 8 | 24,740 | 23,815 | 0.06% | 5,117 | 5 | Township |
| Summit | 9 | 23,357 | 22,625 | 0.18% | 3,897 | 6 | City |
| Roselle | 10 | 23,276 | 22,606 | 0.06% | 8,833 | 3 | Borough |
| Hillside | 11 | 22,857 | 22,333 | -0.01% | 8,261 | 3 | Township |
| Springfield | 12 | 18,913 | 17,106 | 3.11% | 3,667 | 5 | Township |
| Roselle Park | 13 | 16,709 | 13,997 | 7.09% | 13,640 | 1 | Borough |
| Clark | 14 | 16,430 | 15,466 | 0.07% | 3,845 | 4 | Township |
| Berkeley Heights | 15 | 15,685 | 13,210 | 3.6% | 2,522 | 6 | Township |
| New Providence | 16 | 14,091 | 13,729 | -0.01% | 3,821 | 4 | Borough |
| Kenilworth | 17 | 8,608 | 8,393 | 0.07% | 4,009 | 2 | Borough |
| Fanwood | 18 | 8,046 | 7,741 | 0.14% | 6,004 | 1 | Borough |
| Mountainside | 19 | 7,395 | 7,007 | 0.67% | 1,851 | 4 | Borough |
| Union | 20 | 6,723 | 6,502 | 1.11% | 358 | 19 | Township |
| Garwood | 21 | 5,394 | 4,427 | 0.07% | 8,298 | 1 | Borough |
| Springfield | 22 | 3,324 | 3,238 | 0.27% | 113 | 30 | Township |
| Winfield | 23 | 1,451 | 1,415 | 0.07% | 8,017 | 0 | Township |
Elizabeth is the largest city in Union County, with a population of 141,795.
Winfield is the smallest city in Union County, with a population of 1,451.
There are 23 cities in Union County, New Jersey.