Wahkiakum County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 3,978, making it the second-least populous county in Washington. The county seat and only incorporated town is Cathlamet.
The county was formed out of Cowlitz County on April 25, 1854 and is named for Chief Wahkiakum ("Tall Timber") of the Chinook.
According to the U.S. Census Bureau, the county has a total area of 287 square miles (740 km2), of which 263 square miles (680 km2) is land and 24 square miles (62 km2) (8.3%) is water. It is the smallest county of Washington by total area and the third-smallest by land area, ahead of San Juan County and Island County, which are mostly water by area.