Gavin Lambert (23 July 1924 – 17 July 2005) was a British-born screenwriter, novelist and biographer who lived for part of his life in Hollywood. His writing was mainly fiction and nonfiction about the film industry.

Lambert was educated at Cheltenham College and Magdalen College, Oxford, where one of his professors was C. S. Lewis. At Oxford, he befriended Penelope Houston and filmmakers Karel Reisz and Lindsay Anderson, and they founded a short-lived but influential journal, Sequence, which was originally edited by Houston. The magazine, which lasted for only 15 issues, moved to London after the fifth issue, and Lambert and Anderson took over as co-editors. Lambert eventually left Oxford without obtaining a degree. From 1949 to 1956 he edited the journal Sight and Sound, again with Anderson as a regular contributor. At about the same time Lambert was deeply involved in Britain's Free Cinema movement which called for more social realism in contemporary movies. He also wrote film criticism for The Sunday Times and The Guardian. In late 1955, he moved to Hollywood, California, to work as a personal assistant to director Nicholas Ray and worked on the script (uncredited) for Ray's Bigger Than Life (1956). Later, he co-wrote the screenplay for Ray's film Bitter Victory (1957). According to the critic David Robinson, Lambert was Ray's lover for a time.

Lambert became an American citizen in 1964. From 1974 to 1989, he chiefly stayed in Tangier, where he was a close friend of the writer and composer Paul Bowles. He spent the final years of his life in Los Angeles, where he died of pulmonary fibrosis on 17 July 2005. He left behind a brother, niece and nephew, and named Mart Crowley executor of his estate.

Gavin's father's half-sister was Ivy Claudine Godber aka Claudine West (1890–1943), a screenwriter who won an Oscar for her joint writing of the script of Mrs. Miniver in 1942.

His papers are currently housed at the Howard Gotlieb Archival Research Center at Boston University.

Lambert became a notable screenwriter of the Hollywood studio era. In 1954, while still living in England, he wrote his first screenplay, Another Sky, about the sexual awakening of a prim English woman in North Africa. In 1955, he also directed Another Sky in Morocco. This was followed in 1958 by the Hollywood screenplay, Bitter Victory and in 1960 by Sons and Lovers. The latter, for which Lambert gained an Academy Award nomination, is based on a novel by D. H. Lawrence. The Roman Spring of Mrs. Stone (1961) adapted a novella by Tennessee Williams on the affairs of an older actress with a young Italian gigolo. As, from the 1920s through the late 1960s, homosexuality was rarely portrayed on the screen, gay screenwriters like Lambert learned to express their personal sensibilities discreetly between the lines of a film. "The important thing to remember about 'gay influence' in movies," observed Gavin Lambert, "is that it was obviously never direct. It was all subliminal. It couldn't be direct because the mass audience would say, Hey, no way."

But then, in 1965, Lambert adapted his own Hollywood insider novel Inside Daisy Clover (1963) for the screen. Clover, starring Natalie Wood and Robert Redford, which tells the cautionary tale of a teenage movie star involved in the Hollywood studio system of the 1930s and her unhappy marriage to a closeted gay leading man. However, in the film version, he was not fully identified as gay because, at Redford's request, the husband he played was changed from homosexual to appear as though he might be bisexual. From this time on, Lambert and Wood became lifelong friends. Another of Lambert's screenplays was I Never Promised You a Rose Garden (1977), based on a novel by Hannah Green, which describes in layman's terms a teenager's battle with schizophrenia. Later, the author also wrote the scripts for some TV movies such as Second Serve (1986) on transgender tennis player Renée Richards and Liberace: Behind the Music (1988) on gay performer Liberace. In 1997, he contributed to Stephen Frears's film A Personal History of British Cinema. He was heavily quoted in William J. Mann's book, Behind the Screen: How Gays and Lesbians Shaped Hollywood, 1910-1969.

Lambert was also a biographer and novelist, who focused his efforts on biographies of gay and lesbian figures in Hollywood.