State Route 508 (SR 508) is a 32.84-mile (52.85 km) long state highway located in Lewis County within the U.S. state of Washington, extending from an interchange with Interstate 5 (I-5) concurrent with U.S. Route 12 (US 12) in Napavine to SR 7 in Morton. By 1916, a road between Napavine and Cinebar was constructed on the current route of SR 508 and was signed in 1937 as Secondary State Highway 5K (SSH 5K) after being extended to Morton.