Lake Union (Lushootseed: x̌ax̌čuʔ)^[3][a] is a freshwater lake located entirely within the city limits of Seattle, Washington, United States. It is a major part of the Lake Washington Ship Canal, which carries fresh water from the much larger Lake Washington on the east to Puget Sound on the west. The easternmost point of the lake is the Ship Canal Bridge, which carries Interstate 5 over the eastern arm of the lake and separates Lake Union from Portage Bay. Lake Union is the namesake of the neighborhoods located on three of its shores: Eastlake, Westlake and South Lake Union. Notable destinations on the lake include Lake Union Park, the Museum of History & Industry (MOHAI), and the Center for Wooden Boats on the southern shore and Gas Works Park on the northern shore.

The Aurora Bridge (officially the George Washington Memorial Bridge) carries State Route 99 over the western arm of Lake Union. The Aurora Bridge is so named because it carries Aurora Ave N down the western side of the lake. Lake Union's westernmost point can be considered the Fremont Cut, which is located just west of the Aurora Bridge and is spanned by the Fremont Bridge. The Fremont Bridge carries Fremont Ave N between the neighborhoods of Fremont and Queen Anne and separates Lake Union from the rest of the Lake Washington Ship Canal to the west.

Lake Union is a natural lake in Seattle that lies north of the city's central business district. It is fully surrounded by urban neighborhoods and highways. Lake Union is the smaller of the two major lakes in Seattle, the other being Lake Washington.^[4]

Part of the Lake Washington Ship Canal system, water flows into the lake from Lake Washington through the Montlake Cut, and out via the Fremont Cut on its way to Puget Sound. Before the construction of the canal, Lake Union emptied into Salmon Bay via a creek which followed roughly the same course as the Fremont Cut does today.^{[citation needed]}

Because of the connection via the Hiram M. Chittenden Locks to the salt water of Puget Sound, the water is brackish to a certain extent,^[2] which increases in the summer as the inflow rate from Lake Washington decreases and the locks open more frequently for pleasure craft.^[2]

A glacial lake, its basin was dug 12,000 years ago by the Vashon glacier, which also created Lake Washington and Seattle's Green, Bitter, and Haller Lakes.^{[citation needed]}

Several Seattle neighborhoods take their name from the lake: Eastlake, Westlake, Northlake, and South Lake Union; and three major streets are named in relation to it: Westlake Avenue, which runs along its western shore from Downtown to the Fremont Bridge; Eastlake Avenue, which runs along its eastern shore from Cascade to the University District, and Northlake Way, which runs along its northern shore from the University District past Gas Works Park to the edge of Fremont.^{[citation needed]}