Kachess Lake (/kʌtʃis/) is a lake and reservoir along the course of the Kachess River in Kittitas County, Washington, United States. 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 which straddle Interstate 90 north of the Yakima River in the Cascade Range. The other two are Cle Elum Lake, the easternmost which is also north of I-90 and Keechelus Lake, the westernmost, which is south of I-90.

Kachess Lake is part of the Columbia River basin, the Kachess River being a tributary of the Yakima River, which is a 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. The discharge channel for Kachess Reservoir is 2,877 feet long and was constructed from the natural lake to the intake structure of the dam's outlet works, approximately 1800 feet downstream and at a lower elevation than the original lake outlet. The intent of the lowered outlet works was to put all of the average annual runoff into service by adding an additional 76,000 acre feet of natural lake water. As a storage reservoir, Kachess Lake's active capacity is 239,000 acre⋅ft (295 million m).

The name Kachess comes from a Native American term meaning "more fish", in contrast to Keechelus Lake, whose name means "few fish".

• Baker Lake (Alpine Lakes Wilderness)

• Kachess Dam Archived 2007-08-11 at the Wayback Machine, United States Bureau of Reclamation
• U.S. Geological Survey Geographic Names Information System: Kachess Lake, USGS, GNIS
• Historic American Engineering Record (HAER) documentation, filed under Easton, Kittitas County, WA:
  • HAER No. WA-79, "Kachess Dam", 35 photos, 3 measured drawings, 44 data pages, 6 photo caption pages
  • HAER No. WA-79-A, "Kachess Dam, Outlet Channel", 6 photos, 2 photo caption pages
  • HAER No. WA-79-B, "Kachess Dam, 1936 Spillway and Radial Gates", 20 photos, 3 photo caption pages
  • HAER No. WA-79-C, "Kachess Dam, Dike Closing 1913 Spillway", 9 photos, 2 photo caption pages
  • HAER No. WA-79-D, "Kachess Dam, Gatehouse", 24 photos, 3 photo caption pages
  • HAER No. WA-79-E, "Kachess Dam, 1904 Cascade Canal Company Crib Dam", 28 photos, 4 photo caption pages
  • HAER No. WA-79-F, "Kachess Dam, Inlet Channel", 21 photos, 4 photo caption pages
  • HAER No. WA-79-G, "Kachess Dam, Cutoff Channel and Crib Structures", 12 photos, 2 photo caption pages