Elmer Gethin Rees, CBE, FRSE (born 1941) is a mathematician with publications in area 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 Bristol University and the British signals intelligence agency GCHQ, from its creation in 2005 until 2009.
Rees was born in Llandybie and grew up in Wales. He studied at St Catharine's College, Cambridge gaining a B.A. before moving on to Warwick for his Ph.D. His career had taken him to Hull, Institute for Advanced Study, Swansea and St Catherine's College, Oxford before becoming a professor at the University of Edinburgh in 1979 where he remained until retiring from the post in 2005.
He was elected as a fellow of the Royal Society of Edinburgh in 1982. One of his most notable legacies was the establishment of the International Centre for Mathematical Sciences.
Rees was appointed Commander of the Order of the British Empire (CBE) in the 2009 Birthday Honours.