Sir John Leslie Martin (Manchester, 17 August 1908 – 28 July 2000) was an English architect, and a leading advocate of the International Style. Martin's most famous building is the Royal Festival Hall. His work was especially influenced by Alvar Aalto.
After studying at Manchester University Leslie Martin taught at the University of Hull. In 1937 he co-edited with Ben Nicholson and Naum Gabo the journal Circle that reviewed avant-garde abstract art and architecture.
In 1939 Martin, and his wife Sadie Speight, co-wrote The flat book, published by Hutchinson.
During the Second World War Martin was assigned to the pre-nationalisation Railway companies to supervise re-building of bomb damaged regional rail stations. In this capacity Martin developed pre-fabricated designs to speed construction. Following the war Martin was made a Deputy Architect to the London County Council and in 1948 Hugh Casson selected him to lead the design team for the Royal Festival Hall, the most prestigious building project of the Festival of Britain. In part in recognition of his achievement Martin was made Chief Architect of the LCC in 1953 and used his position to promote emerging younger architects Colin St. John Wilson, James Stirling, and Alison and Peter Smithson.
From 1956 Martin was made head of the Architecture School at Cambridge University where Colin St. John Wilson was his assistant. He was Slade Professor of Fine Art at the University of Oxford for 1965–66.