Joan Elisabeth Lowther Murray, MBE (née Clarke; 24 June 1917 – 4 September 1996) was an English cryptanalyst and numismatist who worked as a code-breaker at Bletchley Park during the Second World War.

Although she did not personally seek the spotlight, her role in the Enigma project that decrypted the German secret communications earned her awards and citations, such as appointment as a Member of the Order of the British Empire (MBE), in 1946.

Her work in an undergraduate geometry class at Cambridge drew the attention of mathematician Gordon Welchman, who became her academic supervisor.

[7][3][6] She arrived at Bletchley Park on 17 June 1940 and was initially placed in an all-women group, referred to as "The Girls", who mainly did routine clerical work.

[3] Clarke ended up working with Alan Turing at Bletchley Park in the section known as Hut 8 who she knew slightly through her older brother Michael.

[9] Clarke's first work promotion was to Linguist Grade which was designed to earn her extra money despite the fact that she did not speak another language.

[12][full citation needed] An Enigma machine of four rotors was introduced in 1942 which stymied Hut 8's decryption efforts and led to German U-boats successfully attacking Allied shipping convoys again.

Their work involved decoding German weather signals, enabling Allied bombing raids in the time prior to the Normandy Landings.

[16][17] Shortly after their marriage, John Murray retired from GCHQ due to ill health and the couple moved to Crail in Fife[5] where they lived at Priorscroft, 14 Nethergate.

[3] Recent histories of GCHQ suggest that she played a role in its work during the Falklands War: helping to track the Argentine submarine Santa Fe in April 1982.

[20][21] After meeting her husband, who had published work on the Scottish coinage of the 16th and 17th centuries, Clarke developed an interest in numismatics history.

[23][24] In May 2024 a blue plaque in Joan Clarke's memory was unveiled by English Heritage at 193 Rosendale Road, West Dulwich, London, SE21 8LW, her home as a child.

[6][13][25] Clarke was portrayed by Keira Knightley in the film The Imitation Game (2014), opposite Benedict Cumberbatch as Alan Turing.

Director Morten Tyldum said the film shows how Clarke succeeded in her field despite working in a time "when intelligence wasn't really appreciated in women".