Black Diamond is a city in King County, Washington, United States. The population was 4,697 at the 2020 census. In 2022, with a 35% growth rate, Black Diamond was the fastest growing small city in King County.

The Black Diamond Coal Mining Company was formed in 1861, consolidating the Cumberland and Black Diamond coal mines in the region of Mount Diablo, in Contra Costa County, California. During its years of operation as a mining company, it established three towns: Nortonville, California, Southport, Oregon, and Black Diamond, Washington. The company's mines in California and its settlement of Nortonville later became part of the Black Diamond Mines Regional Park and a California Historical Landmark. Several railroad lines were built in California and Washington to support the company's mines, and the company operated numerous ships to transport its coal. As the mines played out and petroleum became the more common source of energy, the company closed its mines and transitioned into real estate as the Southport Land and Commercial Company.