State Route 104 (SR 104) is a 31.75-mile-long (51.10 km) state highway in the U.S. state of Washington, serving four counties: Jefferson on the Olympic Peninsula, Kitsap on the Kitsap Peninsula, and Snohomish and King in the Puget Sound region. It begins south of Discovery Bay at U.S. Route 101 (US 101) and crosses the Hood Canal Bridge over Hood Canal to the terminus of SR 3 near Port Gamble. SR 104 continues southeast onto the Edmonds–Kingston Ferry to cross the Puget Sound and intersects SR 99 and Interstate 5 (I-5) before ending at SR 522 in Lake Forest Park. SR 104 also has a short spur route that connects the highway to SR 99 at an at-grade signal on the Snohomish–King county line.
Established during the 1964 state highway renumbering, SR 104 was formed out of four highways, themselves created in 1937: Secondary State Highway 9E (SSH 9E) from Discovery Bay to Port Gamble, Primary State Highway 21 (PSH 21) from Port Gamble to Kingston, SSH 1W within Edmonds, and SSH 2B from Edmonds to Lake Forest Park. PSH 21 was originally part of State Road 21, added to the state highway system in 1915 to connect the Port Gamble–Shine ferry to the Kingston ferry landing. The Edmonds–Kingston ferry at the end of State Road 21 has been in operation under various companies since 1923 and was served by the 14-car City of Edmonds. SSH 9E extended from Discovery Bay to Port Ludlow initially and was moved south and extended to the South Point ferry landing in the late 1940s. The bridge replaced the South Point–Lofall ferry when it opened in 1961, but the ferry was re-used during the construction of the second Hood Canal Bridge after the first bridge sank in 1979 and before the new bridge opened in 1982. The Edmonds–Kingston ferry was added to the state ferry system in 1951 before it was incorporated into SR 104 in 1994.