Lake Washington (Lushootseed: x̌ačuʔ) is a large freshwater lake adjacent to the city of Seattle, Washington, United States. It is the largest lake in King County and the second largest natural lake in the state of Washington, after Lake Chelan. It borders the cities of Seattle on the west, Bellevue and Kirkland on the east, Renton on the south, and Kenmore on the north, and encloses Mercer Island. The lake is fed by the Sammamish River at its north end and the Cedar River at its south.
Lake Washington has been known to the Duwamish and other Indigenous peoples living on the lake for millennia as x̌ačuʔ (lit. "lake" in Lushootseed). At the time of European settlement, it was recorded as At-sar-kal in a map sketched by engineer Abiel W. Tinkham; and the Chinook Jargon name, Hyas Chuck ("great/large water"), was also used. Other English names historically used for the lake include Lake Geneva by Isaac N. Ebey; and Lake Duwamish in railroad surveys under Governor Isaac Stevens. Lake Washington received its present name in 1854 after Thomas Mercer suggested it be named after George Washington, as the new Washington Territory had been named the year before.