According to the U.S. Census Bureau, the county has a total area of, of which is land and 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.
Wahkiakum County has the second lowest population of any county in Washington state. Its population of 3,978 is 0.2% the size of the population of Washington's largest county, King. The county has the second highest divorce rate of any Washington county, behind Lincoln County.
As of the census of 2000, there were 3,824 people, 1,553 households, and 1,108 families living in the county. The population density was 14 people per square mile. There were 1,792 housing units at an average density of 7 per square mile. The racial makeup of the county was 93.46% White, 0.26% Black or African American, 1.57% Native American, 0.47% Asian, 0.08% Pacific Islander, 1.65% from other races, and 2.51% from two or more races. 2.56% of the population were Hispanic or Latino of any race. 18.7% were of German, 13.1% Norwegian, 10.6% United States or American, 9.1% English, 7.7% Irish, 6.5% Swedish, and 5.9% Finnish ancestry. 96.7% spoke English and 2.5% Spanish as a first language. There were 1,553 households out of which 26.90% had children under the age of 18 living with them, 61.40% were married couplesliving together, 6.30% had a female householder with no husband present, and 28.60% were non-families. 24.40% of all households were made up of individuals and 10.60% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 2.83. In the county, the population was spread out with 23.40% under the age of 18, 5.30% from 18 to 24, 22.20% from 25 to 44, 30.60% from 45 to 64, and 18.50% who were 65 years of age or older. The median age was 44 years. For every 100 females there were 100.10 males. For every 100 females age 18 and over, there were 98.10 males. The median income for a household in the county was $39,444, and the median income for a family was $47,604. Males had a median income of $37,123 versus $27,938 for females. The per capita income for the county was $19,063. About 5.90% of families and 8.10% of the population were below the poverty line, including 11.00% of those under age 18 and 2.70% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 3,978 people, 1,737 households, and 1,187 families living in the county. The population density was. There were 2,067 housing units at an average density of. The racial makeup of the county was 94.0% white, 1.3% American Indian, 0.6% Asian, 0.3% black or African American, 0.2% Pacific islander, 0.7% from other races, and 3.1% from two or more races. Those of Hispanic or Latino origin made up 2.7% of the population. In terms of ancestry, 19.8% were Norwegian, 19.3% were German, 13.3% were English, 9.6% were American, 8.5% were Swedish, and 6.8% were Irish. Of the 1,737 households, 21.9% had children under the age of 18 living with them, 57.2% were married couples living together, 6.2% had a female householder with no husband present, 31.7% were non-families, and 26.8% of all households were made up of individuals. The average household size was 2.26 and the average family size was 2.69. The median age was 52.3 years. The median income for a household in the county was $40,372 and the median income for a family was $47,266. Males had a median income of $44,779 versus $36,111 for females. The per capita income for the county was $23,115. About 7.1% of families and 12.2% of the population were below the poverty line, including 14.5% of those under age 18 and 10.7% of those age 65 or over.
