Existing research suggests that international peacekeeping contributes to conflict resolution and helps sustain peace, often in locations with hostile ethnic divisions. However, it is unclear whether the presence of peacekeepers actually reduces underlying ethnocentric views and parochial behaviors that sustain those divisions. We examine the effects of NATO peacekeeper deployments on ethnocentrism in postwar Bosnia. While peacekeepers were not randomly deployed in Bosnia, we find that highly ethnocentric attitudes were common across Bosnia at the onset of peacekeeper deployments, reducing endogeneity concerns. To measure ethnocentrism, we employ a variety of survey instruments as well as a behavioral experiment (the dictator game) with ethnic treatments across time. We find that regions with peacekeepers exhibit lower levels of ethnocentrism in comparison to regions without peacekeepers, and this effect persists even after peacekeepers have departed. The peacekeeping effect is also robust to a sub-sample of ethnic Bosnian Serbs, suggesting that peacekeeper deployments can have positive effects on diminishing ethnocentrism, even when local communities are especially hostile to their presence. Our results speak to the potential long-term role of peacekeepers in reducing tensions among groups in conflict.