Hillsboro ( HILZ-burr-oh) is a city in the U.S. state of Oregon and is the county seat of Washington County. Situated in the Tualatin Valley on the west side of the Portland metropolitan area, the city hosts many high-technology companies, such as Intel, locally known as the Silicon Forest. The population was 106,447 at the 2020 census, making Hillsboro the fifth-most populous city in Oregon.

Settlers founded a community here in 1842, later named after David Hill, an Oregon politician. Transportation by riverboat on the Tualatin River was part of Hillsboro's settler economy. A railroad reached the area in the early 1870s and an interurban electric railway about four decades later. These railways, as well as highways, aided the slow growth of the city to about 2,000 people by 1910 and about 5,000 by 1950, before the arrival of high-tech companies in the 1980s.

Hillsboro has a council-manager government consisting of a city manager and a city council headed by a mayor. In addition to high-tech industry, sectors important to Hillsboro's economy are health care, retail sales, and agriculture, including grapes and wineries. The city operates more than twenty parks and the mixed-use Hillsboro Stadium, and ten sites in the city are listed on the National Register of Historic Places (NRHP). Modes of transportation include private vehicles, public buses and light rail, and aircraft using the Hillsboro Airport. The city is home to Pacific University's Health Professions Campus.