Mercer Island is a city in King County, Washington, United States, located on an island of the same name in the southern portion of Lake Washington. Mercer Island is in the Seattle metropolitan area, with Seattle to its west and Bellevue to its east.
Mercer Island is connected to the mainland on both sides by bridges carrying Interstate 90, with the city of Seattle to the west and the city of Bellevue to the east. The Lacey V. Murrow Memorial Bridge and the parallel Homer M. Hadley Memorial Bridge are floating bridges that span Lake Washington and carry, respectively, eastbound and westbound lanes of Interstate 90 and connect Mercer Island to the northern portion of Seattle's South End. I-90 traverses the northern portion of Mercer Island and is then carried from the island to Bellevue over the East Channel of Lake Washington by the East Channel Bridge. Mercer Island is located closer to Bellevue than it is to Seattle, and is therefore often considered to be part of King County's Eastside.
The population was 22,699 at the 2010 census and an estimated 25,894 in 2019. Mercer Island is the most populated island in a lake within the United States.