Mercer Island is a city in King County, Washington, United States, located upon an island of the same name in the southern allowance 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 upon both sides by bridges carrying Interstate 90. The Lacey V. Murrow Memorial Bridge and the parallel Homer M. Hadley Memorial Bridge are wandering bridges that span Lake Washington and carry, respectively, eastbound and westbound lanes of Interstate 90 and border Mercer Island to the northern allowance of Seattle’s South End. I-90 traverses the northern ration of Mercer Island and is then carried from the island to Bellevue more than 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 in view of that 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.^{[better source needed]} The ZIP code 98040 is unique to Mercer Island. Mercer Island has the fifth highest per-capita pension in the acknowledge of Washington and is one of the 100 richest ZIP codes in the USA according to the IRS figures for Adjusted rude Income.

The western side of the island was home to two Snoqualmie villages prior to white deal in the Puget Sound region. Mercer Island, named for the Mercer relations of Seattle, was first arranged between 1870 and 1880. The Mercer brothers often rowed amongst the island and Seattle to choose berries, hunt, and fish. Those brothers, Thomas Mercer and Asa Mercer, were members of the Mercer relations of Virginia. The first large settlement, East Seattle, was toward the northwest side of the island—near the McGilvara neighborhood. During 1889, a C C Calkins built a large and gilded resort, the Calkins Hotel. The hotel was reached via steamboat between Madison Park, Leschi Park, and the Eastside. Guests included President Benjamin Harrison, of 1901, amongst other booming dignitaries from Seattle to the East Coast of the United States. Burned by a obscure fire, the hotel was razed during 1908.^{[citation needed]}

The Calkins Landing continued serve and presumably aided the instigation of a more remaining population. A denser urban community with thing district developed toward the central northern island between the McGilvra neighborhood and Luther Burbank Park. This community now composes the majority of the island’s crest through the Middle Island neighborhood.^{[citation needed]}