| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Butler | 1 | 16,894 | 17,207 | -0.05% | 781 | 22 | Township |
| Adams | 2 | 16,156 | 14,854 | 1.44% | 723 | 22 | Township |
| Butler | 3 | 12,752 | 13,484 | -0.99% | 4,690 | 3 | City |
| Butler | 4 | 10,405 | 9,544 | 1.17% | 309 | 34 | Township |
| Slippery Rock | 5 | 8,325 | 6,692 | -0.29% | 323 | 26 | Township |
| Buffalo | 6 | 8,248 | 7,923 | 0.41% | 342 | 24 | Township |
| Center | 7 | 7,774 | 7,890 | -0.05% | 319 | 24 | Township |
| Middlesex | 8 | 7,588 | 6,892 | 1% | 328 | 23 | Township |
| Butler | 9 | 5,189 | 4,881 | 1.65% | 199 | 26 | Township |
| Summit | 10 | 4,277 | 4,486 | -0.72% | 191 | 22 | Township |
| Connoquenessing | 11 | 4,124 | 4,257 | -0.65% | 181 | 23 | Township |
| Zelienople | 12 | 3,717 | 3,912 | -0.93% | 1,811 | 2 | Borough |
| Slippery Rock | 13 | 3,648 | 3,183 | 2.73% | 2,218 | 2 | Borough |
| Forward | 14 | 3,460 | 3,196 | 1.41% | 150 | 23 | Township |
| Harmony | 15 | 3,065 | 3,157 | -0.2% | 1,054 | 3 | Township |
| Slippery Rock | 16 | 2,848 | 2,906 | -0.11% | 96 | 30 | Township |
| Oakland | 17 | 2,699 | 2,778 | -0.3% | 114 | 24 | Township |
| Butler | 18 | 2,637 | 2,554 | 0.42% | 110 | 24 | Township |
| Clay | 19 | 2,464 | 2,541 | -0.36% | 98 | 25 | Township |
| Muddy Creek | 20 | 2,164 | 2,207 | -0.18% | 96 | 23 | Township |
| Evans City | 21 | 1,654 | 1,746 | -0.96% | 2,022 | 1 | Borough |
| Mars | 22 | 1,379 | 1,447 | -0.86% | 2,934 | 0 | Borough |
| Saxonburg | 23 | 1,355 | 1,428 | -0.95% | 1,496 | 1 | Borough |
| Worth | 24 | 1,353 | 1,397 | -0.29% | 54 | 25 | Township |
| Brady | 25 | 1,178 | 1,219 | -0.51% | 69 | 17 | Township |
| Prospect | 26 | 1,077 | 1,129 | -0.83% | 264 | 4 | Borough |
| Cherry | 27 | 1,043 | 1,048 | 0.19% | 41 | 26 | Township |
| Harmony | 28 | 896 | 944 | -0.99% | 2,274 | 0 | Borough |
| Chicora | 29 | 862 | 908 | -0.92% | 1,617 | 1 | Borough |
| Connoquenessing | 30 | 836 | 679 | 3.34% | 617 | 1 | Borough |
| Harrisville | 31 | 775 | 815 | -0.9% | 964 | 1 | Borough |
| East Butler | 32 | 721 | 762 | -0.96% | 691 | 1 | Borough |
| Valencia | 33 | 717 | 542 | 5.29% | 2,109 | 0 | Borough |
| Allegheny | 34 | 605 | 618 | 0% | 24 | 25 | Township |
| Harmony | 35 | 518 | 512 | 0.78% | 17 | 31 | Township |
| Harmony | 36 | 433 | 465 | -1.14% | 13 | 34 | Township |
| Bruin | 37 | 396 | 421 | -1.25% | 217 | 2 | Borough |
| Callery | 38 | 331 | 352 | -1.19% | 616 | 1 | Borough |
| West Liberty | 39 | 317 | 334 | -0.94% | 83 | 4 | Borough |
| Eau Claire | 40 | 287 | 305 | -1.03% | 225 | 1 | Borough |
| Portersville | 41 | 218 | 230 | -0.91% | 274 | 1 | Borough |
| Barkeyville | 42 | 182 | 188 | -0.55% | 55 | 3 | Borough |
| West Sunbury | 43 | 172 | 182 | -1.15% | 1,564 | 0 | Borough |
| Petrolia | 44 | 169 | 179 | -1.17% | 428 | 0 | Borough |
| Karns City | 45 | 158 | 167 | -1.25% | 435 | 0 | Borough |
Butler is the largest city in Butler County, with a population of 16,894.
Karns City is the smallest city in Butler County, with a population of 158.
There are 45 cities in Butler County, Pennsylvania.