In this paper we use assemblage thinking to offer a new interrogation of the relationalities of volunteering and development and to revisit volunteering’s relationship to cosmopolitanism. Recent debates about the rise of new actors in development cooperation have seen a growing interest in the geopolitical significance of volunteers and their contribution to development. Research has addressed the ways international volunteering can shape cosmopolitan subjectivities, whilst claims for volunteering’s universality are a key feature of global development policy. However, we argue that existing approaches to volunteering, cosmopolitanism and development remain contained by established development imaginaries and their ascription of agency, authority and expertise to actors from the global North. We use the idea of the assemblage, and data from two research projects, the International Federation of Red Cross and Red Crescent’s (IFRC) Global Review on Volunteering, and a doctoral research project on diaspora volunteering, to explore the constitution of what volunteering is within and between places. Through this, we identify alternative sites for interrogating the capacity of volunteering to challenge established ideas of agency, care and responsibility in development.