Geraint Francis Lewis, FLSW (born 14 March 1969) is a Welsh astrophysicist, best known for his work on dark energy, gravitational lensing and galactic cannibalism.

Lewis won the 2016 Walter Boas Medal in recognition of excellence in research in physics.

[2] In 2021, he was awarded the David Allen Prize of the Astronomical Society of Australia for exceptional achievement in astronomy communication.

In 2002, Lewis joined the University of Sydney, where he is currently the head of the Gravitational Astrophysics Group.

Another aspect of Lewis's research uses the phenomenon of gravitational lensing to probe the nature and distribution of the pervasive dark matter, and employing individual stars to magnify the hearts of quasars, the most luminous objects in the universe.