The content of group-based emotions like collective nostalgia (i.e., a sentimental longing for how one’s social group used to be) matters. Yet this has been distressingly overlooked despite the predictive utility that the content of a specific group-based emotion may yield. For instance, there is growing evidence that collective nostalgia for a society that is more homogenous heightens prejudice, while collective nostalgia for a society that is more open and tolerant society increases acceptance of outgroups. In an intervention-based longitudinal study with Polish adolescents (N = 476), we tested whether the content of collective nostalgia can be influenced by highlighting particular aspects of Polish history. The intervention highlighted a history of co-existence between ingroup and outgroup members. We predicted and found that the intervention increased openness-focused nostalgia, which in turn improved intergroup attitudes. Results suggest that community-based interventions can improve intergroup attitudes by eliciting nostalgia for a more open past.