King County Courthouse, County courthouse in downtown Seattle, United States

The King County Courthouse is a government building at 516 Third Avenue in downtown Seattle, just north of City Hall Park. It began in 1914 as a five-story steel frame and reinforced concrete structure, dedicated May 4, 1916 as the City-County Building. Six floors were added in 1930 by architects Henry Bittman and John L. McCauley, followed by a three-story attic.

King County voters first rejected, then approved, a new government building in 1911 on land once owned by Seattle founder Henry Yesler. Construction began in 1914 on a five-story steel-frame and reinforced-concrete structure, dedicated May 4, 1916 as the City-County Building. Architects Henry Bittman and John L. McCauley added six floors in 1930, and a three-story attic followed. The city moved out in 1962, when the building was renamed the King County Courthouse.

The courthouse houses the King County Law Library, which serves as a research hub for legal professionals and the general public seeking to understand the law. This library draws people interested in accessing court records and legal information that shape decisions affecting the region.

The courthouse stands at 516 Third Avenue, between Dilling Way and James Street, just north of City Hall Park. Since December 2019 the 3rd Avenue entrance has been closed to most people for security reasons, with limited access for those needing accommodation. An enclosed skybridge links the building to the King County Jail, and a pedestrian tunnel connects it to the King County Administration Building.

The building connects to neighboring government structures through underground tunnels and sky bridges, including a direct link to the county jail. These hidden passages allow for secure movement of detainees between court appearances without public visibility.