Kittitas County is a county located in the U.S. state of Washington. At the 2020 census, its population was 44,337. Its county seat and largest city is Ellensburg. The county was created in November 1883 when it was carved out of Yakima County. Kittitas County comprises the Ellensburg, Washington, Micropolitan Statistical Area. There are numerous interpretations of the county's name, which is from the language of the Yakama Nation.