Cathlamet is a town located along the Ocean Beach Highway in Wahkiakum County, Washington, United States, where it is the county seat. The population was 560 at the 2020 census.

State Route 4 (SR 4) is a 62.27-mile-long (100.21 km) state highway, serving the southwestern region of the U.S. state of Washington. The highway, also known as Ocean Beach Highway, travels east along the Columbia River from U.S. Route 101 (US 101) at Johnston's Landing through Pacific, Wahkiakum and Cowlitz counties to an interchange with Interstate 5 (I-5) in Kelso. SR 4 is designated as part of the Lewis and Clark Trail Scenic Byway, a state scenic byway, as well as part of the National Highway System.