Harry St John Bridger Philby, (3 April 1885 – 30 September 1960), also known as Jack Philby or Sheikh Abdullah, was a British Arabist, adviser, explorer, writer, and Colonial Office intelligence officer.

As he states in his autobiography, he “became something of a fanatic” and in 1908 “the first Socialist to join the Indian Civil Service”. After studying Oriental languages at the University of Cambridge, he was posted to Lahore in the Punjab in 1908, acquiring fluency in Urdu, Punjabi, Baluchi, Persian and eventually Arabic. He converted to Islam in 1930 and later became an adviser to Ibn Saud, urging him to unite the Arabian Peninsula under Saudi rule, and helping him to negotiate with the United Kingdom and the United States when petroleum was discovered in 1938; in addition he married for the second time, to a Saudi Arabian.

His only son by his first wife Dora Johnston was Kim Philby, who became known worldwide as a double agent for the Soviet Union, where he defected in 1963. One of his three sons with his second wife Rozy al-Abdul Aziz is the former United Nations Resident Coordinator in, inter-alia, Kuwait and later Turkmenistan, Khaled Philby.