Harold Kainalu Long Castle (July 3, 1886 – August 19, 1967)[1] was a landowner, real estate developer, and later a philanthropist in Hawaii.

Harold Kainalu Long Castle was born July 3, 1886, in Honolulu.

In 1917, he purchased almost 10,000 acres (40 km2) of land on the windward side of the island of Oahu, in what was then the ahupuaʻa of Kailua, to establish his sprawling Kaneohe Ranch.

[2] He and his foundation have donated large amounts of land to educational and other public institutions, including Hawaii Loa College, ʻIolani School, Castle High School, Kainalu Elementary School, Castle Medical Center, and the Mokapu peninsula land that became Marine Corps Base Hawaii.

[2] Plantation Estate, his home in Kailua, Honolulu County, Hawaii, was used by Barack Obama as a Winter White House during Christmas vacations in 2008, 2009, and 2010.