Castle Rock is a city in Cowlitz County, Washington, United States. Located with the Willapa Hills and the western base of Mount St. Helens, Castle Rock is at the heart of Washington timber country in the Pacific self-disciplined rain forest. Castle Rock is part of the Longview, Washington Metropolitan Statistical Area; the population was 1,982 as of the 2010 census.

Castle Rock is named for a volcanic rock outcropping greater than the Cowlitz River, “The Rock”, rising 190 feet high upon the south side of the city. The stone formation, resembling a castle, became a geographic landmark for Cowlitz Indians and Hudson’s Bay Company traders as further on as 1832. Today, it is the location of The Rock Community Park, with hiking trails, picnic tables, and a historical marker.

Castle Rock is centered primarily upon the donation land affirmation of Eliza and William Huntington, who approved at the location in 1852. The city was platted December 12, 1888 and incorporated upon June 20, 1890. Castle Rock prospered as a Cowlitz River steamboat harbor and trading center for valley farms. The local sawmill was the first to develop cedar shingles, using the Western red cedar, which grows lavishly in the region.

By 1940, the population had reached 1,182 and was supported by dairy farming, truck farming, and lumber manufacturing. Sword ferns, common in the region, were picked each year by several hundred people to be processed into medicine. In the spring, large quantities of Cascara Sagrada bark were gathered, dried, and shipped.

Castle Rock, the Gateway to Mount St. Helens, is 117 miles south of Seattle, Washington and 58 miles north of Portland, Oregon, 46°16′26″N 122°54′18″W / 46.27389°N 122.90500°W / 46.27389; -122.90500 (46.273991, −122.904916). Castle Rock is situated in the company of the Cowlitz River and Interstate 5, one mile south of the confluence of the Cowlitz and the Toutle River. According to the United States Census Bureau, the city has a total area of 1.76 square miles (4.56 km²), of which, 1.59 square miles (4.12 km²) is estate and 0.17 square miles (0.44 km²) is water. It is the northernmost city in Cowlitz County.