Karel Reisz (born 1926, Ostrava, Czechoslovakia, died London, United Kingdom, 2002) was a Jewish refugee who became one of the most important film-makers in post war Britain.

Reisz joined the Royal Air Force towards the end of the war, after the death of his parents at Auschwitz. After the war, he studied Natural Sciences at Cambridge, and began to write for film journals, including Sight and Sound. He co-founded Sequence with Lindsay Anderson and Gavin Lambert in 1947.

He was also a founder member of the Free Cinema documentary movement. His 1959 film We Are the Lambeth Boys was a naturalistic depiction of the members of a South London boys' club, which was unusual in showing the life of working-class teenagers as it was, with skiffle music and cigarettes intact.

His first feature film Saturday Night and Sunday Morning (1960) was based on a social realist novel by Alan Sillitoe, and used many of the same techniques as his earlier documentaries. In particular, scenes filmed at the Raleigh factory in Nottingham have the now familiar look of a documentary, and give the story a vivid sense of verisimillitude.

He produced This Sporting Life (1963), and directed Morgan: A Suitable Case For Treatment (1966), Isadora (1968), The Gambler (1974), Who'll Stop the Rain (1978), The French Lieutenant's Woman (1981), Sweet Dreams (1985), and Everybody Wins (1990) among others, and was a patron of the British Film Institute.