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. The population was 25,748 at the 2020 census. The ZIP code 98040 is unique to Mercer Island. Mercer Island has the fifth highest per-capita income in the state of Washington and is one of the 100 richest ZIP codes in the USA according to the IRS figures for Adjusted Gross Income.