(Peter John Landin, 5 June 1930, Sheffield – 3 June 2009) was a British computer scientist. He was one of the first to realize that the lambda calculus could be used to model a programming language, an insight that is essential to development of both functional programming and denotational semantics.
Landin, who was bisexual, became involved with the Gay Liberation Front (GLF) during the early 1970s. He was once arrested as part of a GLF demonstration. He was a dedicated cyclist and moved around London on his bike until it became physically impossible for him to do so.