Western Marxism is a current of Marxist theory that arose from Western and Central Europe in the aftermath of the 1917 October Revolution in Russia and the ascent of Leninism. The term denotes a loose collection of theorists who advanced an interpretation of Marxism distinct from classical and Orthodox Marxism and the Marxism–Leninism of the Soviet Union.

Less concerned with economic analysis than earlier schools of Marxist thought, Western Marxism placed greater emphasis on the study of the cultural trends of capitalist society, deploying the more philosophical and subjective aspects of Marxism, and incorporating non-Marxist approaches to investigating culture and historical development. Key themes included the influence of Georg Wilhelm Friedrich Hegel on Karl Marx's thought, and the recovery of the "Young Marx," emphasizing his early, humanistic writings.

While some early Western Marxists were prominent political activists, Western Marxism became predominantly the reserve of university-based philosophers. Since the 1960s, the concept has been closely associated with the New Left. Many Western Marxists were adherents of Marxist humanism, but the term also encompasses figures and schools of thought that were strongly critical of humanism and the dialectics of Hegel.

In the 1920s, the Third International disparagingly branded certain Marxists of the period as "West European" theorists. By 1930, one such figure, Karl Korsch, had begun to refer to himself as a "Western Communist". Maurice Merleau-Ponty popularized the term Western Marxism with his book Adventures of the Dialectic in 1955. Merleau-Ponty delineated a body of Marxist thought starting with György Lukács that differs from both the Soviet interpretation of Marxism and the earlier Marxism of the Second International.

Perry Anderson notes that Western Marxism was born from the failure of proletarian revolutions in various advanced capitalist societies in Western Europe – Germany, Austria, Hungary, and Italy – in the wake of the First World War. He argues that the tradition represents a divorce between socialist theory and working-class practice that resulted from the defeat and stagnation of the Western working class after 1920.

Western Marxism traces its origins to 1923, when György Lukács's History and Class Consciousness and Karl Korsch's Marxism and Philosophy were published. In these books, Lukács and Korsch proffer a Marxism that underlines the Hegelian basis of Marx's thought. They argue that Marxism is not simply a theory of political economy that improves on its bourgeois predecessors. Nor is it a scientific sociology, akin to the natural sciences. For them, Marxism is primarily a critique – a self-conscious transformation of society. They stipulate that Marxism does not make philosophy obsolete, as "vulgar" Marxism believes; instead, Marxism preserves the truths of philosophy until their revolutionary transformation into reality.

Their work was met with hostility by the Third International, which saw Marxism as a universal science of history and nature. Nonetheless, this style of Marxism was taken up by Germany's Frankfurt School in the 1930s. The Prison Notebooks of the Italian Communist Antonio Gramsci, written during this period, but not published until much later, are also classified as belonging to Western Marxism. Ernst Bloch is a contemporaneous figure who is likewise sometimes judged to be one of Western Marxism's founding fathers.

After the Second World War, a French Western Marxism was constituted by theorists based around the journals Arguments, Les Temps Modernes and Socialisme ou Barbarie such as Lucien Goldmann, Henri Lefebvre, Maurice Merleau-Ponty, and Jean-Paul Sartre. This later generation of Western Marxists were overwhelmingly professional academics and frequently professors of philosophy.