Martin John Rees, Baron Rees of Ludlow is a British cosmologist and astrophysicist. He has been Astronomer Royal since 1995 and Master of Trinity College, Cambridge from 2004 to 2012. He was President of the Royal Society between 2005 and 2010.

Rees was educated at Trinity College, Cambridge (where he attained a first class degree in mathematics), and completed his doctorate under Dennis Sciama at Cambridge.

After holding post-doctoral research positions in the United Kingdom and the United States, he taught at Sussex University and the University of Cambridge, where he was the Plumian Professor until 1991, and the director of the Institute of Astronomy. From 1992 to 2003, he was Royal Society Research Professor, and from 2003 Professor of Cosmology and Astrophysics. He was Professor of Astronomy at Gresham College, London, in 1975 and became a Fellow of the Royal Society in 1979. He also holds Visiting Professorships at Imperial College London and at the University of Leicester and is an Honorary Fellow of Darwin College, Cambridge and Jesus College, Cambridge. He has received honorary degrees from a number of universities including Sussex, Uppsala, Toronto, Durham, Oxford, Yale, Melbourne and Sydney. He belongs to several foreign academies, including the US National Academy of Sciences, the Russian Academy of Sciences and the Pontifical Academy of Sciences. He has been President of the Royal Astronomical Society (1992–94) and the British Association (1995–96), and was a Member of Council of the Royal Institution of Great Britain until 2010. Rees is the author of more than 500 research papers, and he has made important contributions to the origin of cosmic microwave background radiation, as well as to galaxy clustering and formation. His studies of the distribution of quasars led to final disproof of Steady State theory.

His books include Before the Beginning, Our Final Century?, Just Six Numbers, Our Cosmic Habitat, Gravity's Fatal Attraction, and From Here to Infinity: Scientific Horizons, an expanded version of his BBC Reith Lectures. A further book, What We Still Don't Know, is forthcoming.

Ever since his book Before the BeginningOur Final Century? was published, he has been concerned with the threats stemming from humanity's ever-heavier 'footprint' on the global environment, and with the runaway consequences of ever more powerful technologies. These concerns led him to join with colleagues in setting up a Centre for the Study of Existential Risks (CSER). This is based in Cambridge but has a strong international advisory board.

As well as expanding his scientific interests, Rees has written and spoken extensively about the problems and challenges of the 21st century, and the interfaces between science, ethics and politics. He is a member of the Board of the Princeton Institute for Advanced Study, the IPPR, the Oxford Martin School and the Gates Cambridge Trust. He is also one of three founders of the Centre for the Study of Existential Risk. He has formerly been a Trustee of the British Museum and the Science Museum.

Rees’ honors and awards include the Heineman Prize, the Gold Medal of the Royal Astronomical Society, the Balzan Prize for High Energy Astrophysics, a Knight Bachelor, a Bruce Medal, the Bruno Rossi Prize , the Gruber Prize in Cosmology, the Albert Einstein World Award of Science, the Henry Norris Russell Lectureship of the American Astronomical Society, the Lifeboat Foundation's Guardian Award, the Royal Society's Michael Faraday Prize for science communication, the Crafoord Prize, an Order of Merit from the Queen, the Caird Medal of the National Maritime Museum, the Templeton Prize and Isaac Newton Medal. He also has an asteroid (Asteroid 4587 Rees) named after him.