Marxist humanism had a profound influence on the political, social, and cultural life of the socialist countries and on left movements throughout the globe during the 1960s and 1970s. This paper seeks to contribute to a critical and comparative reassessment of the Marxist-humanist turn in the socialist countries and their impact on intellectual and social life during the post-Stalinist period in Eastern Europe and beyond. While mapping the theoretical contributions of the humanist turn and its leading concepts—alienation, praxis, and the “holistically developed person,” among others—the paper also examines the disturbing political directions that it took. It traces the peculiar convergence between Marxist humanism and ethnonationalism during the post-Stalinist period in socialist Bulgaria. Studying the formation of the ethnonationalist doctrine from the post-Stalinist period, the paper shows how the ethnic assimilation projects launched by the socialist government during the last three decades of socialism unfolded within a Marxist-humanist framework.