|Location||Kittitas County, Washington, United States|
|Type||reservoir, natural lake|
|Primary inflows||Kachess River|
|Primary outflows||Kachess River|
|Catchment area||63 sq mi (160 km2)|
|Basin countries||United States|
|Max. length||9.6 mi (15.4 km)|
|Max. width||1.2 mi (1.9 km)|
|Water volume||238,000 acre feet (294,000,000 m3)|
|Surface elevation||2,254 ft (687 m)|
Kachess Lake is a lake and reservoir along the course of the Kachess River in Washington state, US. The upper part of the lake, north of a narrows, is called Little Kachess Lake. The Kachess River flows into the lake from the north, and out from the south. Kachess Lake is the middle of the three large lakes north of Interstate 90 and the Yakima River in the Cascade Range, the other two being Keechelus Lake and Cle Elum Lake.
Kachess Lake is part of the Columbia River basin, the Kachess River being a tributary of the Yakima River, which is tributary to the Columbia River.
The lake is used as a storage reservoir for the Yakima Project, an irrigation project run by the United States Bureau of Reclamation. Although a natural lake, Kachess Lake's capacity and discharge is controlled by Kachess Dam, a 115 foot (35 m) high earthfill structure built in 1912. As a storage reservoir, Kachess Lake's active capacity is 239,000 acre feet (295,000,000 m3).
The name Kachess comes from an Indian term meaning "more fish", in contrast to Keechelus Lake, whose name means "few fish".