Masahito, Prince Hitachi is a member of the Imperial House of Japan, the younger brother of Emperor emeritus Akihito and the paternal uncle of Emperor Naruhito. He is the second son and sixth born child of Emperor Shōwa and Empress Kōjun and is third and last in line to the Chrysanthemum Throne. He is mainly known for philanthropic activities and his research on the causes of cancer.

The Imperial House , also referred to as the Imperial Family and the House of Yamato, is the imperial family of Japan, consisting of those members of the extended family of the reigning Emperor of Japan who undertake official and public duties. Under the present Constitution of Japan, the emperor is "the symbol of the State and of the unity of the people". Other members of the Imperial Family perform ceremonial and social duties, but have no role in the affairs of government. The duties as an emperor are passed down the line to their male children. This Japanese monarchy is the oldest continuous hereditary monarchy in the world. The Imperial House recognizes 126 monarchs, beginning with Emperor Jimmu, and continuing up to the current emperor, Naruhito. However, scholars have agreed that there is no evidence of Jimmu's existence, that the traditional narrative of Japan's founding is mythical, and that Jimmu is a mythical figure. Historical evidence for the first 25 emperors is mythical, but there is sufficient evidence of an unbroken agnatic line since the early 6th century. Historically, verifiable emperors of Japan start from AD 539 with Emperor Kinmei.