Histories of monism have generally ended with the First World War and placed it within the context of the technocratic fantasies of liberal supporters of antipolitical Kultur in late Wilhelmine Germany. This article argues instead that monism achieved its widest practical dispersal during the Weimar Republic in the socialist milieu. It follows the path of liberal intellectuals from opposition to war and monarchy into the socialist movements, where they took leading positions in local government, union educational institutions, and the expanding universe of socialist cultural associations. There they sought to revise Marxism to bring it in line with their theories of biological and sociological evolution. The article follows key four areas of the socialist workers’ culture movement and examines how monism shaped the theories and practices of sex reform, free body culture, festival culture and educational innovation. It thereby demonstrates for the first time the central role of secularist dissent and monist worldview in some of the iconic utopian projects of interwar socialism.