Robert J. Stewart (16 February 1877 – 1971) was a member of the Communist Party of Great Britain (CPGB) and was in charge of the underground cell which, in the 1930s, operated a clandestine transmitter in Wimbledon that relayed information between the CPGB and the Comintern in Moscow. He was the CPGB's spymaster and, at one stage, controlled the Cambridge Five.[1]