In the 1920s and 1930s, Latin American radicals engaged in urgent debates over how to combat racism, resist empire, and reimagine the nation-state. Drawing on a global array of sources, Radical Sovereignty reconstructs these transnational discussions that unfolded in such far-flung locations as Mexico City, Buenos Aires, Havana, Moscow, and Brussels. Energized by the Mexican and Russian Revolutions, communists, trade unionists, peasant organizers, and anti-imperial activists emerged from these debates with innovative ideas for addressing historical oppressions, including proposals for a pan-continental confederation and calls to grant black and indigenous peoples in the Americas the right to form their own sovereign states. While these projects did not come to fruition, they left an enduring mark on Latin America’s political landscape, bequeathing approaches to race, ethnicity, and self-determination that have resurfaced in recent years.