Claude Lefort (born 1924) is a French philosopher and activist.
He was politically active by 1942 under the influence of his tutor, the phenomenologist Maurice Merleau-Ponty  (whose posthumous publications Lefort later edited ). By 1943 he was organising a faction of the Trotskyist Parti Communiste Internationaliste at the Lycée Henri IV in Paris.
Lefort was impressed by Cornelius Castoriadis when he first met him. From 1946 he collaborated with him in the Chaulieu-Montal Tendency, so called from their pseudonyms Pierre Chaulieu (Castoriadis) and Claude Montal (Lefort). They published On the Regime and Against the Defence of the USSR, a critique of both the Soviet Union and its Trotskyist supporters. They suggested that the USSR was dominated by a social layer of bureaucrats, and that it consisted of a new kind of society as aggressive as Western European societies. By 1948, having tried to persuade other Trotskyists of their viewpoint, they broke away with about a dozen others and founded the libertarian socialist group Socialisme ou Barbarie. Lefort's text L'Expérience prolétarienne was important in shifting the group's focus towards forms of self-organisation.
For a time Lefort wrote for both the Socialisme ou Barbarie journal and for Les Temps Modernes. His involvement in the latter journal ended after a published debate during 1952-4 over Sartre's article The Communists and Peace.
Lefort was for a long time uncomfortable with Socialisme ou Barbarie's "organisationalist" tendencies. In 1958 he, Henri Simon and others left and formed Informations et Liaison Ouvrières.
In his academic career, Lefort taught at the University of São Paulo, at the Sorbonne and at the École des Hautes Études en Sciences Sociales, being affiliated to the Centre de recherches politiques Raymond Aron. He has written on the early political writers Niccolò Machiavelli and Étienne de La Boétie and explored "the Totalitarian enterprise" in its "denial of social division... [and] of the difference between the order of power, the order of law and the order of knowledge" (Philosopher?, 1985).