Newcastle is an Eastside city in King County, Washington, United States. Its population was 10,380 at the 2010 census.
Although Newcastle was not incorporated until 1994, it has been an important settlement and town since the late 19th century and played a major role in the development of Seattle and the Seattle region. Newcastle was one of the region's earliest 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.
The Newcastle coal mine began producing coal by the 1870s. More than 13 million tons of coal had been extracted by the time the mine closed in 1963. Newcastle's coal mining legacy, and the mines many Chinese laborers, remains in place names such as Coal Creek, Coal Creek Parkway, and China Creek Golf Course.