12 cities
| City | Rank | 2025 Pop. ↓ | 2020 Pop. | Annual Change | Density | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Hillsboro | 1 | 111,150 | 107,084 | 0.74% | 4,203 | 26 | City |
| Beaverton | 2 | 98,488 | 97,558 | 0.19% | 5,014 | 20 | City |
| Tigard | 3 | 57,867 | 55,034 | 0.99% | 4,537 | 13 | City |
| Tualatin | 4 | 28,074 | 27,884 | 0.14% | 3,343 | 8 | City |
| Forest Grove | 5 | 27,323 | 26,088 | 0.91% | 4,638 | 6 | City |
| Sherwood | 6 | 20,438 | 20,452 | -0.01% | 4,176 | 5 | City |
| Cornelius | 7 | 16,008 | 12,813 | 4.16% | 6,844 | 2 | City |
| King City | 8 | 4,958 | 5,176 | -0.88% | 6,113 | 1 | City |
| North Plains | 9 | 3,419 | 3,443 | -0.15% | 3,137 | 1 | City |
| Durham | 10 | 1,864 | 1,944 | -0.85% | 4,546 | 0 | City |
| Banks | 11 | 1,829 | 1,838 | -0.11% | 2,400 | 1 | City |
| Gaston | 12 | 675 | 680 | -0.15% | 1,979 | 0 | City |
Hillsboro is the largest city in Washington County, with a population of 111,150.
Gaston is the smallest city in Washington County, with a population of 675.
There are 12 cities in Washington County, Oregon.