William Klein (April 19, 1926 – September 10, 2022) was an American-French photographer and filmmaker noted for his ironic approach to both media and his extensive use of unusual photographic techniques in the context of photojournalism and fashion photography. He was ranked 25th on Professional Photographer's list of 100 most influential photographers.

Klein trained as a painter, studying under Fernand Léger, and found early success with exhibitions of his work. He soon moved on to photography and achieved widespread fame as a fashion photographer for Vogue and for his photo essays on various cities. He directed feature-length fiction films, numerous short and feature-length documentaries and produced over 250 television commercials.

He was awarded the Prix Nadar in 1957, the Royal Photographic Society's Centenary Medal and Honorary Fellowship (HonFRPS) in 1999, and the Outstanding Contribution to Photography Award at the Sony World Photography Awards in 2011.

A retrospective exhibition of his work, William Klein: YES: Photographs, Paintings, Films, 1948–2013, was shown at the International Center of Photography in New York until September 15, 2022.

Klein was born in New York City into an impoverished Jewish family. He graduated from high school early and enrolled at the City College of New York at the age of 14 to study sociology. He joined the U.S. Army during World War II, and was stationed in Germany and later France with the 2nd Armored Division, where he permanently settled after being discharged in 1948.

In 1948, Klein enrolled at the Sorbonne, and later studied with Fernand Léger. At the time, Klein was interested in abstract painting and sculpture. In 1952, he had two successful solo exhibitions in Milan and began a collaboration with the architect Angelo Mangiarotti. Klein also experimented with kinetic art, and it was at an exhibition of his kinetic sculptures that he met Alexander Liberman, the art director for Vogue.

He moved on to photography and achieved widespread fame as a fashion photographer for Vogue and for his photo essays on various cities. Despite having no formal training as a photographer, Klein won the Prix Nadar in 1957 for New York, a book of photographs taken during a brief return to his hometown in 1954. Klein's work was considered revolutionary for its "ambivalent and ironic approach to the world of fashion", its "uncompromising rejection of the then prevailing rules of photography" and for his extensive use of wide-angle and telephoto lenses, natural lighting and motion blur. The New York Times' Katherine Knorr writes that, along with Robert Frank, Klein is considered "among the fathers of street photography, one of those mixed compliments that classifies a man who is hard to classify."

Klein's most popular photographic works are Gun 1, New York (1955), The Holy family on bike (Rome, 1956), Cineposter (Tokyo, 1961), Vogue (fashion models in the streets of New York, Rome and Paris for Vogue magazine, 1963), Love on the Beat (Serge Gainsbourg album sleeve, 1984), Club Allegro Fortissimo (1990) and Autoportrait (a book of painted contact prints, 1995).