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'Richard of Conisburgh, 3rd Earl of Cambridge' (c. 1375 – 5 August 1415) was the younger son of Edmund of Langley, 1st Duke of York and Isabella of Castile, Duchess of York.
His paternal grandparents were Edward III of England and Philippa of Hainault. His maternal grandparents were Pedro of Castile and María de Padilla who was of Jewish descent. (Peggy K. Liss, "Isabel the Queen," New York: Oxford University Press, 1992, p. 165; James Reston, Jr. "Dogs of God," New York: Doubleday, p. 18.)
He was born at Conisburgh Castle in Yorkshire, and was confirmed in the Earldom of Cambridge, which had been resigned by his brother, in 1414. In about 1406, he married his cousin, Anne Mortimer, also a descendant of Edward III (his great great granddaughter), through his son Lionel of Antwerp. A papal dispensation was dated for 28 May 1406, making it most likely that the marriage took place in May or June. It was through her that the Yorkist faction in the Wars of the Roses claimed the throne. Their marriage produced a daughter, Isabel Plantagenet, and a son, Richard Plantagenet, 3rd Duke of York. He later laid claim on the throne, beginning the Wars of the Roses.
It is believed that Anne died giving birth to Richard. Following Anne's death, Cambridge married Matilda Clifford.
He was discovered to be one of the fomentors of the Southampton Plot against King Henry V immediately prior to departure on the French campaign. (His elder brother, Edward of Norwich, 2nd Duke of York, would die at the Battle of Agincourt, less than three months later.) He was stripped of all his titles and estates and was executed on 5 August 1415 at Southampton Green, Hampshire, England; before the fleet set sail on 11 August 1415.