John Cairncross (July 25, 1913 – October 8, 1995) was a British intelligence officer during World War II who, along with four other men (Kim Philby, Donald Maclean, Guy Burgess and Anthony Blunt) passed secrets to the Soviet Union during the war. This group became known as the Cambridge Five.

Cairncross was educated at Glasgow University and Trinity College, Cambridge, where he studied modern languages. After graduating, he worked in the Foreign Office. In 1937 he joined the Communist party. In 1942 he worked on ciphers at Bletchley Park and MI6. During this time, he passed documents through secret channels to the Soviet Union. The information he supplied enabled the Soviets to keep their ciphers one step ahead of British Intelligence.

Cairncross admitted to spying in 1951 after MI5 found incriminating papers in his possession. Some believe that the information he supplied about the Western atomic weapons programmes kick-started the Soviet nuclear programme. He was never prosecuted, however, which later led to charges that the government engaged in a conspiracy to cover up his role. Indeed, the identity of the infamous "fifth man" in the Cambridge Five remained a mystery until 1990, when KGB defectors Yuri Modin and Oleg Gordievsky fingered Cairncross.

After his confession, Cairncross moved to Rome where he worked for the United Nations Food and Agricultural Organization until he retired to the south of France. John Cairncross died in 1995.

He was the brother of economist Sir Alexander Kirkland Cairncross (Alec Cairncross) and the uncle of journalist Frances Cairncross.