Sir John (Jack) Harold Plumb FBA (20 August 1911 – 21 October 2001) was a British historian, known for his books on British 18th-century history.
During the Second World War, Plumb worked in the codebreaking department of the Foreign Office at Bletchley Park, Hut 8 and Hut 4, later Block B. He headed a section working on a German Navy hand cipher, Reservehandverfahren.