In the last decades of the twentieth century, globalization, technological changes, and neoliberal reforms weakened labor organizations in Latin America and beyond. In this context, scholars not only documented labor union decline but also suggested that historical differences between formal-sector insiders (e.g., salaried workers) and outsiders (e.g., informal sector, rural, and unemployed workers) would deepen. However, unexpected alliances between labor unions representing insiders and social movements representing outsiders have since formed in a number of countries. Some of these alliances have crystallized into new national labor federations or overarching associations, which I call insider-outsider coalitions. I argue that these coalitions emerge in the context of major political or economic threats affecting labor movements. When labor movements split in response to such threats, defecting labor unions seek outsider allies. Coalition formation succeeds when defecting unions are strong and there are existing networks or organizations among outsiders. The chapter illustrates the argument of insider-outsider coalition formation in the case of Argentina.