Richard John Nelmes (born 1943) is a British crystallographer, currently professor emeritus and Honorary Professorial Fellow in the School of Physics and Astronomy at the University of Edinburgh.

As chair of the university's Centre for Science at Extreme Conditions (CSEC), he studies the behaviour of crystal structures and materials such as ice, methane, and silicon at pressures of up to 1 million times normal atmospheric pressure.

[3] This work has many industrial applications, from creating new superconductors to cooking foods using pressure instead of heat.

[3] Nelmes also works at the Rutherford Appleton Laboratory where he helped to establish the Diamond Light Source X-ray synchrotron and has pioneered high-pressure X-ray diffraction.

[1] In 2007, he received the Duddell Medal from the Institute of Physics for "pioneering new techniques and instrumentation that have transformed high-pressure structural science, including the production of quantitative diffraction data that can be analysed to pressures beyond a megabar".