Elmer Gethin Rees, (19 November 1941 – 4 October 2019) was a Welsh mathematician with publications in areas ranging from topology, differential geometry, algebraic geometry, linear algebra and Morse theory to robotics.
He held the post of Director of the Heilbronn Institute for Mathematical Research, a partnership between the University of Bristol and the British signals intelligence agency GCHQ, from its creation in 2005 until 2009.