This dissertation examines how European and American banking institutions catering to middle- and working-class people sought to mobilize their capital to challenge the predominant model of international financial capitalism during the interwar years. Focusing on four sets of financial institutions whose identities were intimately linked with the “popular” clientele they served—savings banks, cooperative banks, trade union banks, and their communist analogues—I chart how influential actors within these institutions engaged in transnational efforts to challenge the entrenched position of private banks in international finance, though in pursuit of divergent political and commercial objectives. Drawing upon politicized attitudes towards popular capital and motivated by the opportunities and pressures of post-World War I internationalism, they attempted to build parallel institutional channels that could mobilize the modest financial assets of the masses to compete with or even displace capitalist banking. This study reconstructs formal and informal networks of debate and activism in which savings bankers, cooperators, and labor activists developed projects for international financial action based on popular ownership and participation. To uncover these networks, I draw on an eclectic range of sources including national and international archives, periodicals produced by syndical, cooperative, communist, and savings bank movements, and private correspondence of American and European leaders in these movements. I argue for a more capacious understanding of the political valence of financialization in this period. Far from being accepted as a neutral outcome of economic development, lamented as a depoliticizing penetration of capitalist logic into the social life of the masses, or turned to narrowly nationalistic ends, popular financial ownership—whether by individuals or institutions claiming to represent them—was recognized as a foundation on which to enact transnational solidarity. However, the political content of this solidarity varied considerably between these projects of international popular finance. Some of them sought to moralize capitalism within liberal or fascist political structures, while others aimed to strengthen cooperative or socialist alternatives. My dissertation presents an institutional history of projects of popular financial power and their limits which will be of interest to scholars of modern Europe and the U.S., international institutions, transnationalism, and global capitalism. I also hope to offer historical perspective on ongoing debates about the potential for collective action by workers, consumers, and investors in our own financialized era.