Socialisme ou Barbarie (SouB; "Socialism or Barbarism") was a French-based radical libertarian socialist group of the post-World War II period whose name comes from a phrase which was misattributed to Friedrich Engels by Rosa Luxemburg in the Junius Pamphlet, but which probably was most likely first used by Karl Kautsky.

The group originated in the Trotskyist Fourth International, where Castoriadis and Claude Lefort constituted a Chaulieu–Montal tendency in the French Parti Communiste Internationaliste in 1946.

In 1948, they experienced their "final disenchantment with Trotskyism",[5] leading them to break away to form Socialisme ou Barbarie, whose journal began appearing in March 1949.

The group was composed of both intellectuals and workers, and agreed with the idea that the main enemies of society were the bureaucracies which governed modern capitalism.

Socialisme ou Barbarie was critical of Leninism, rejecting the idea of a revolutionary party,[7] and placing an emphasis on the importance of workers' councils.

After the May 1958 crisis and an influx of new meeting attendees, disagreements on the organisational role of a political group led to the departure of some prominent members including Claude Lefort and Henri Simon to form Informations et Liaison Ouvrières.

The main Socialisme ou Barbarie journal continued publishing until a final edition in 1965, after which the group became dormant and announced its indefinite suspension in June 1967.