Between the years 1870 and 1914, leftist intellectuals in the Kingdom of Serbia theorized and promoted a project of Balkan Federation as a strategic priority in the social, economic, and political transformation of the region. This article offers a genealogy of these federalist ideas and places them in dialogue with rival projects of regional unification in the Balkans and Eastern Europe during the long nineteenth century. It begins by developing a typology of federalist projects in Europe, categorizing these according to the underlying models of sovereignty upon which they were founded. I identify four categories: revolutionary-republican, imperial-reformist, imperial-irredentist, and revolutionary-social. Instead of organizing these federalisms according to their authors’ ideological commitments (socialist, nationalist, pan-Slavic) or their geographic scope (Balkan, Danubian), the article argues that examining their respective models of sovereignty offers intellectual historians a more productive approach to identify the unexpected convergences and divergences of federalist projects during this period. The article then moves into a discussion of the development of Serbian socialist ideas of Balkan Federation, beginning first with the work of Svetozar Marković (1846–1875) and then turning to the writings of the fin de siècle Social Democratic Party in the decade before World War I. Situating this genealogy of socialist Balkan federalism in its broader European intellectual milieu, I use the above typology to identify the ways in which Serbian socialists converged and diverged from contemporary federalist projects, including the reformist ideas of the Austro-Marxists, the irredentist strategy of the Serbian Progressive Party, and the republican ideas of Karel Kautsky.