The Tualatin River defines the landscape of Oregon's most populous Washington County

Countries & Cities · Published 15 October 2026 Swipe more like this →

In short: While many assume the most populous Washington County sits near the nation's capital, the largest one is actually in Oregon. Home to over 600,000 residents, this county is defined by the Tualatin River and a unique economic transformation that turned agricultural plains into the heart of the state's Silicon Forest.

Originally established as Twality District in 1843, the region was renamed for the first U.S. president in 1849. Its geography is dominated by the Tualatin Valley, a fertile basin cradled by the Northern Oregon Coast Range, the Chehalem Mountains, and the Tualatin Mountains. The Tualatin River flows through this landscape, serving as the primary waterway, while Henry Hagg Lake stands as the largest lake in the county. Despite its sprawling urban growth, only about 0.3% of the county's 726 square miles is covered by water.

The county's economic identity shifted dramatically during the 1980s and 1990s. Once a region defined by orchards and vineyards, it became the epicenter of Oregon's Silicon Forest. Major technology firms, including Intel, which maintains its largest concentration of employees in Hillsboro, anchor the local economy. This industrial evolution is complemented by the presence of global headquarters for corporations like Nike and Columbia Sportswear. These developments have helped the county grow into the second most populous in Oregon, with a 2020 census population of 600,372.

Political shifts have mirrored this rapid urbanization. For much of its early history, the county was a reliable Republican stronghold, consistently supporting the party's presidential nominees from 1860 until 1928. However, the influence of increasing urbanization and changing national party dynamics led to a significant realignment. Since the 1990s, the county has trended toward the Democratic Party, with Joe Biden securing 65.5% of the vote in 2020. Today, the county balances its high-tech industrial base with ongoing urban planning efforts designed to provide city-level services to its unincorporated communities.

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