State Route 506 (SR 506) is an 11.53-mile (18.56 km) long state highway in Cowlitz and Lewis counties, part of the U.S. state of Washington, serving the communities of Ryderwood and Vader. SR 506 begins at the intersection of 2nd and Morse Street in Ryderwood, a community in Cowlitz County, traveling north into Lewis County and turning east past Vader to parallel the Cowlitz River and interchange Interstate 5 (I-5) twice west of Toledo.