Gauche Socialiste is a Trotskyist faction within Quebec Solidaire (formerly the Parti de la Democratie Socialiste (PDS) in Quebec, Canada). It was formed in 1983 by Trotskyists who left or were expelled from the Revolutionary Workers League/Ligue Ouvrière Révolutionnaire when the group turned away from Trotskyism in the early 1980s. Gauche Socialiste members had previously been in the Organisation Combat Socialiste, which existed from 1980 to 1982, and were briefly part of the Mouvement socialiste, which was founded in 1981.
Gauche Socialiste is the Quebec section of the reunified Fourth International. The group publishes the periodical La Gauche. The group's counterpart in English Canada was Socialist Challenge which later joined the New Socialist Group and forms a Fourth International Caucus within it.
Gauche Socialiste joined with several other organizations to form the Union des forces progressistes (UFP) in 2002. Gauche Socialiste is now involved in Quebec Solidaire. Quebec Solidaire was formed in February 2006 in a merger of UFP and Option Citoyenne.