Lake Union 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.

Seattle is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2022 population of 749,256 it is the most populous city in both the state of Washington and the Pacific Northwest region of North America, and the 18th-most populous city in the United States. The Seattle metropolitan area's population is 4.02 million, making it the 15th-largest in the United States. Its growth rate of 21.1% between 2010 and 2020 made it one of the country's fastest-growing large cities.