12 cities
| City | Rank | 2026 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Hillsboro | 1 | 111,963 | 107,084 | 0.73% | 4,234 | 26 | City |
| Beaverton | 2 | 98,674 | 97,558 | 0.19% | 5,024 | 20 | City |
| Tigard | 3 | 58,434 | 55,034 | 0.98% | 4,581 | 13 | City |
| Tualatin | 4 | 28,112 | 27,884 | 0.14% | 3,347 | 8 | City |
| Forest Grove | 5 | 27,570 | 26,088 | 0.9% | 4,680 | 6 | City |
| Sherwood | 6 | 20,435 | 20,452 | -0.01% | 4,176 | 5 | City |
| Cornelius | 7 | 16,647 | 12,813 | 3.99% | 7,117 | 2 | City |
| King City | 8 | 4,915 | 5,176 | -0.87% | 6,060 | 1 | City |
| North Plains | 9 | 3,414 | 3,443 | -0.15% | 3,132 | 1 | City |
| Durham | 10 | 1,848 | 1,944 | -0.86% | 4,507 | 0 | City |
| Banks | 11 | 1,827 | 1,838 | -0.11% | 2,398 | 1 | City |
| Gaston | 12 | 674 | 680 | -0.15% | 1,977 | 0 | City |
Hillsboro is the largest city in Washington County, with a population of 111,963.
Gaston is the smallest city in Washington County, with a population of 674.
There are 12 cities in Washington County, Oregon.