High Quality Content by WIKIPEDIA articles! Hillsboro is the fifth-largest city in the U.S. state of Oregon and is the county seat of Washington County. Lying in the Tualatin Valley on the west side of the Portland metropolitan area, the city is home to many high-technology (high-tech) companies, such as Intel, that compose what has become known as the Silicon Forest. As of the 2010 Census, the city`s population was 91,611.