As of 2000, the population is 33,362. Its county seat is Ellensburg, which is also the county's largest city.

There are numerous interpretations of the name, which is of Kittitas Indian origin. According to Jennifer Cochran, it "has been said to mean everything from 'white chalk' to 'shale rock' to 'shoal people' to 'land of plenty'. Most anthropologists and historians concede that each interpretation has some validity depending upon the particular dialect spoken."

Kittitas County was created out of Klickitat County on November 24, 1883.