Although effective interventions to increase international collective action for human rights are highly desirable, the validation of theory-based interventions and their transfer to this practical field is still scarce. We investigated whether collective action intention can be improved by using a diversity intervention technique rooted in social psychology. The intervention builds on the ingroup projection model, postulating that negative intergroup relations are based on the perception of ingroups as more typical of a common superordinate group than outgroups (i.e., relative prototypicality). Thereby, the (quasi-)experimental study tested the ingroup projection model’s theoretical assumptions in the context of Model United Nations (MUN) conferences. We hypothesized that the diversity intervention leads to a higher perceived diversity within the superordinate group (the United Nations, UN) as well as identification with the superordinate group (UN). Furthermore, we hypothesized an indirect effect of the intervention on collective action intention mediated by perception of diversity of, and identification with, the superordinate group. In comparison to the control group (n = 45), those participants who received the diversity workshop intervention (n = 55) perceived less relative prototypicality and more diversity of the UN. In addition, we provide evidence of a serial mediation: Compared to the control group, the diversity workshop group perceived the UN as relatively more diverse, facilitating identification with the UN. In turn, this was associated with a stronger intention to act collectively. This study shows the importance of including psychological theories in the field of international relations.