AbstractResearch shows that peacekeepers reduce conflict intensity; however, effects of deployment on nonpolitical violence are unknown. This article focuses on criminal violence and proposes a twofold mechanism to explain why peacekeeping missions, even when effectively reducing conflict, can inadvertently increase criminal violence. First, less conflict opens up economic opportunities (so‐called peacekeeping economies) and provides operational security for organized crime, thus increasing violent competition among criminal groups. Second, demobilized combatants are vulnerable to turn to crime because of limited legal livelihood opportunities and their training in warfare. While UN troops may exacerbate these dynamics, UN police's peculiar role is likely to successfully contain criminal violence. Cross‐national and subnational empirical analyses show that large UN military deployments result in higher homicide rates, whereas UN police, overall, moderate this collateral effect.