There are 39 counties in Washington. As of 2019, Washington's most populous county is King County, home to Seattle, with a population of 2,188,649. This represents a population growth of 12.97% since the 2010 census. King County is followed by Piece County (876,764), Snohomish County (801,633), and Spokane County (506,152) in terms of population. These are the only counties with a population of more than 500,000. All four counties have posted significant growth since the census ranging from 7% to almost 13%.
The least populous Washington county is Garfield County with a population of 2,210. This represents a population decline of 2.26% since the last census. Other sparsely populated counties in Washington include Columbia County (4,047), Wahkiakum County (4,264), and Ferry County (7,594). These are the only counties in Washington with a population under 10,000. Three of these counties reported a population decline since 2010. Wahkiakum County had population growth of 7.05% since the last census.
Franklin County has boasted the highest population growth rate in Washington since the last census, growing 16.48% to 92,125. Four other cities have experienced population growth above 12%: King County (12.97%), Kittitas County (12.74%), Benton County (12.30%), and Snohomish County (12.05%). The county with the largest negative growth is Garfield County at -2.26%.