Simon Lehna Singh, MBE (Order of the British Empire) (born 1 January 1964) is a British (United Kingdom) author who has specialised in writing about mathematical (mathematics) and scientific (science) topics in an accessible manner. He is the youngest of three brothers, his eldest brother being Tom Singh, the founder of the UK New Look (New Look (store)) chain of stores.

His written works include Fermats Last Theorem (Fermats Last Theorem (book)) (in the United States titled Fermats Enigma: The Epic Quest to Solve the Worlds Greatest Mathematical Problem), The Code Book (about cryptography and its history (history of cryptography)), Big Bang (Big Bang (book)) (about the Big Bang theory and the origins of the universe) and Trick or Treatment? Alternative Medicine on Trial (Trick or Treatment) (about complementary and alternative medicine).

He has also produced documentaries and works for television to accompany his books, is a trustee of NESTA, the National Museum of Science and Industry and co-founded the Undergraduate Ambassadors Scheme.

In 2008, Singh was unsuccessfully sued for libel by the British Chiropractic Association for criticising their activities in a column in The Guardian. A "furious backlash (Backlash (sociology))" to the now dropped lawsuit has resulted in the filing of formal complaints of false advertising against more than 500 individual chiropractors within one 24 hour period, one national chiropractic organisation ordering its members to take down their websites, and Nature Medicine noting that the case had gathered wide support for Singh, as well as prompting calls for the reform of English libel laws. On 1 April 2010, Simon Singh won his court appeal for the right to rely on the defence of fair comment. On 15 April 2010, the BCA officially withdrew its lawsuit, ending the case.