Neosocialism was a political trend of socialism, represented in France during the 1930s and in Belgium, which included several revisionist tendencies in the French Section of the Workers' International (SFIO). In the wake of the Great Depression, a group of right-wing members, led by Henri de Man in Belgium, founder of planisme, and in France Marcel Déat, Pierre Renaudel, René Belin, the "neo-Turks" of the Radical-Socialist Party (Pierre Mendès-France, etc.), opposed themselves both to Marxism and to gradual reformism. Instead, influenced by Henri de Man's planisme, they promoted a "constructive revolution" headed by the state and technocrats, through economic planification. Such ideas also influenced the Non-Conformist Movement in the French right-wing.
Marcel Déat published in 1930 Perspectives socialistes (Socialist Perspectives), a revisionist work closely influenced by Henri de Man's planisme. Along with over a hundred articles written in La Vie Socialiste, the review of the SFIO's right-wing, Perspective socialistes marked the shift of Déat from classical Socialism to Neo-Socialism. Déat replaced class struggle by collaboration of classes and national solidarity, advocated corporatism as a social organization model, replaced the notion of "Socialism" by "anti-Capitalism" and supported an authoritarian state which would plan the economy and from which parliamentarism would be repealed.
The Neo-Socialist faction inside of the SFIO, which included Marcel Déat and Pierre Renaudel, were expelled during the November 1933 Congress, because of their revisionist stances and admiration for Fascism. The neos advocated alliances with the middle-classes and favored making compromises with the "bourgeois" Radical-Socialist Party to enact the SFIO's program one issue at a time. After having been expelled from the SFIO, Marcel Déat and his followers created the Parti socialiste de France-Union Jean Jaurès (1933–1935) which was one of the main expression of Neo-Socialism in France. Inside the Confédération générale du travail (CGT) trade union, Neo-Socialism was represented by René Belin's Syndicats (then Redressements)'s faction.
On the other hand, Henri de Man's planisme influenced the left-wing of the Radical Party, called "Young Turks" (among them Pierre Mendès France). Planisme would later influence dirigisme, semi-planified economy, regionalism, spatial planning as well as Mendesism, "left-wing Gaullism" (Louis Vallon) and Socialist clubs in the 1960s (Club Jean Moulin, etc.).
The Neo-Socialists, however, evolved toward a form of participatory and nationalist socialism which eventually led them to join with the reactionary right and support the collaborationist Vichy Regime during the Second World War (René Belin and Marcel Déat became members of the Vichy government). As a result, Déat's neo-socialism was discredited in France after the war.