Kittitas County /ˈkɪtɨtæs/ is a county located in the U.S. state of Washington. As of the 2010 census, the population was 40,915. Its county seat and largest city is Ellensburg. The county was created on November 24, 1883 when it separated from Yakima County.There are numerous interpretations of the name, which is from the language of the Yakama Nation. According to one source, it "has been said to mean everything from 'white chalk' to 'shale rock' to 'shoal people' to 'land of plenty'.