Simon Lehna Singh, MBE (born 19 September 1964) is a British popular science author and theoretical and particle physicist. His written works include Fermat's Last Theorem (in the United States titled Fermat's Enigma: The Epic Quest to Solve the World's Greatest Mathematical Problem), The Code Book (about cryptography and its history), Big Bang (about the Big Bang theory and the origins of the universe), Trick or Treatment? Alternative Medicine on Trial (about complementary and alternative medicine, co-written by Edzard Ernst) and The Simpsons and Their Mathematical Secrets (about mathematical ideas and theorems hidden in episodes of The Simpsons and Futurama). In 2012 Singh founded the Good Thinking Society, through which he created the website "Parallel" to help students learn mathematics.

Singh has also produced documentaries and works for television to accompany his books, is a trustee of the National Museum of Science and Industry, a patron of Humanists UK, founder of the Good Thinking Society, and co-founder of the Undergraduate Ambassadors Scheme.

Singh was born in a Sikh family to parents who emigrated from Punjab, India to Britain in 1950. He is the youngest of three brothers, his eldest brother being Tom Singh, the founder of the UK New Look chain of stores. Singh grew up in Wellington, Somerset, attending Wellington School, and went on to Imperial College London, where he studied physics. He was active in the student union, becoming President of the Royal College of Science Union. Later he completed a PhD in particle physics at the University of Cambridge as a postgraduate student of Emmanuel College, Cambridge while working at CERN, Geneva.

In 1983, he was part of the UA2 experiment in CERN. In 1987, Singh taught science at The Doon School, an independent all-boys' boarding school in India. In 1990 Singh returned to England and joined the BBC's Science and Features Department, where he was a producer and director working on programmes such as Tomorrow's World and Horizon. Singh was introduced to Richard Wiseman through their collaboration on Tomorrow's World. At Wiseman's suggestion, Singh directed a segment about politicians lying in different mediums, and getting the public's opinion on whether the person was lying or not.

After attending some of Wiseman's lectures, Singh came up with the idea to create a show together, and Theatre of Science was born. It was a way to deliver science to normal people in an entertaining manner. Richard Wiseman has influenced Singh in such a way that Singh states:

My writing initially was about pure science but a lot of my research now has been inspired by his desire to debunk things such as the paranormal – we both hate psychics, mediums, pseudoscience in general.

Singh directed his BAFTA award-winning documentary about the world's most notorious mathematical problem entitled Fermat's Last Theorem in 1996. The film was memorable for its opening shot of a middle-aged mathematician, Andrew Wiles, holding back tears as he recalled the moment when he finally realised how to resolve the fundamental error in his proof of Fermat's Last Theorem. The documentary was originally transmitted in January 1996 as an edition of the BBC Horizon series. It was also aired in America as part of the NOVA series. The Proof, as it was re-titled, was nominated for an Emmy Award.

The story of this celebrated mathematical problem was also the subject of Singh's first book, Fermat's Last Theorem. In 1997, he began working on his second book, The Code Book, a history of codes and codebreaking. As well as explaining the science of codes and describing the impact of cryptography on history, the book also contends that cryptography is more important today than ever before. The Code Book has resulted in a return to television for him. He presented The Science of Secrecy, a five-part series for Channel 4. The stories in the series range from the cipher that sealed the fate of Mary, Queen of Scots, to the coded Zimmermann Telegram that changed the course of the First World War. Other programmes discuss how two great 19th-century geniuses raced to decipher Egyptian hieroglyphs and how modern encryption can guarantee privacy on the Internet.