State Route 504 (SR 504, designated as the Spirit Lake Memorial Highway) is a state highway in southwestern Washington state in the United States. It travels along the North Fork Toutle River to the Mount St. Helens area, serving as the main access to the Mount St. Helens National Volcanic Monument. The highway begins at an interchange with Interstate 5 (I-5) and SR 411 in Castle Rock and terminates at the Johnston Ridge Observatory near Spirit Lake.