Jonathan R. Ellis is a British theoretical physicist who is currently Clerk Maxwell Professor of Theoretical Physics at King's College London. He attended Cambridge University, earning his Ph.D. in theoretical high-energy particle physics in 1971 and has held a contract at CERN since 1978. He was awarded the Maxwell Medal and the Paul Dirac Prize by the Institute of Physics in 1982 and 2005 respectively, and is an Elected Fellow of the Royal Society of London since 1985 and of the Institute of Physics since 1991. He was awarded an Honorary Doctorate from the University of Southampton, and twice won the First Award in the Gravity Research Foundation essay competition (in 1999 and 2005). He is also Honorary Doctor at Uppsala University.

Ellis' research interests focus on the phenomenological aspects of particle physics, though he has also made important contributions to astrophysics, cosmology and quantum gravity. Most of his publications relate directly to experiment, from interpreting measurements and the results of searches for new particles, to exploring the physics that could be done with future accelerators. He was one of the pioneers of research at the interface between particle physics and cosmology, which has since become a sub-specialty of its own: particle astrophysics.