Geraint F. Lewis (born 14 March 1969) is a Welsh astrophysicist, who is best known for his work on dark energy, gravitational lensing and galactic cannibalism.Lewis is a Professor of Astrophysics (Teaching and Research) at the Sydney Institute for Astronomy, part of the University of Sydney's School of Physics. He is head of the Gravitational Astrophysics Group. He was previously the Associate Head for Research at the School of Physics, and held an Australian Research Council Future Fellowship between 2011 and 2015. Lewis won the 2016 Walter Boas Medal in recognition of excellence in research in Physics.