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.

The Yakima River is a tributary of the Columbia River in south central and eastern Washington state, named for the indigenous Yakama people. Lewis and Clark mention in their journals that the Chin-nâm pam called the river Tâpe têtt, possibly from the French tape-tête, meaning "head hit". The length of the river from headwaters to mouth is 214 miles (344 km), with an average drop of 9.85 feet per mile (1.866 m/km). It is the longest river entirely in Washington state.