Karl Duncker (Leipzig, 2 February 1903 – 23 February 1940) was a German psychologist.

Until 1935 he was a student and assistant of the founders of Gestalt psychology in Berlin: Max Wertheimer, Wolfgang Köhler and Kurt Koffka. In 1935, exiled by the Nazis, he got an assistantship at Cambridge University with Frederic Bartlett. Later he emigrated to the USA where he was again an assistant of Köhler’s at Swarthmore College. He committed suicide in 1940 at 37 years of age. He had been suffering from depression for some time and had received professional treatment.