Newcastle is an Eastside city in King County, Washington, United States. The population was 13,017 at the 2020 census. Newcastle was one of the region's first coal mining areas and its railroad link to Seattle was the first in King County. Timber also played a role in the early history of Newcastle. Coal delivered by rail from Newcastle's mines to Seattle fueled the growth of the Port of Seattle and attracted railroads, most notably the Great Northern Railway.

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Based on per capita income, Newcastle ranks 11th of 522 areas in the state of Washington to be ranked. In CNN Money's 2011 rankings of best places to live in the United States, Newcastle was in the top 20, along with nearby Sammamish. It ranked 17th in Newsweek's 2009 rankings of best places to live in the United States.