Nicholas Kemmer FRS FRSE (7 December 1911 – 21 October 1998) was a Russian-born nuclear physicist working in Britain, who played an integral and leading edge role in United Kingdom's nuclear programme, and was known as a mentor of Abdus Salam – a Nobel laureate in physics.

He received his doctorate in nuclear physics at the University of Zurich and worked as an assistant to Wolfgang Pauli, who had to give strong arguments in 1936, before being allowed to employ a non-Swiss national.

Kemmer moved to Trinity College, Cambridge in 1940 to work on Tube Alloys, the wartime atomic energy project.

In 1940, when Egon Bretscher and Norman Feather showed that a slow neutron reactor fuelled with uranium would in theory produce substantial amounts of plutonium-239 as a by-product, Kemmer (who was lodging at the Bretschers') proposed the names Neptunium for the new element 93 and Plutonium for 94 by analogy with the outer planets Neptune and Pluto beyond Uranus (uranium being element 92).

The Americans Edwin M. McMillan and Philip Abelson at the Berkeley Radiation Laboratory, who had made the same discovery, fortuitously suggested the same names.