Washington State Route 203

State Route 203
A map of the Seattle region with SR 203 highlighted in red
Route information
Auxiliary route of US 2
Maintained by WSDOT
Length	24.28 mi[1] (39.07 km)
Existed	1964–present
Major junctions
South end	SR 202 in Fall City
North end	US 2 in Monroe
Location
Country	United States
State	Washington
Counties	King, Snohomish
Highway system
State highways in Washington Interstate US State Scenic Pre-1964 1964 renumbering Former
← SR 202 → SR 204

State Route 203 (SR 203) is a state highway in the U.S. state of Washington that traverses part of King and Snohomish counties. It runs north–south for 24 miles (39 km) through the Snoqualmie Valley, connecting Fall City, Carnation, Duvall, and Monroe. The highway terminates to the south at a roundabout with SR 202 near Fall City and to the north at a junction with U.S. Route 2 (US 2) in Monroe.

The road network along the Snoqualmie River developed in the early 20th century to connect communities that had been established near a branch of the Chicago, Milwaukee, St. Paul and Pacific Railroad. These county roads were paved and added to the state highway system in 1937 as Secondary State Highway 15B (SSH 15B), which served as a connector between state highways in Fall City and Monroe.

SSH 15B was straightened and rebuilt by the state government in the 1940s and 1950s before being replaced by SR 203 during the 1964 state highway renumbering. SR 203 was included as part of the proposed corridor for an outer freeway bypass, tentatively named Interstate 605, in the early 2000s. The highway was later improved by a series of construction projects, including the construction of two roundabouts in Fall City and Duvall.