What are the security consequences of population movements? This article seeks to provide a better understanding of when, how, and under what conditions terrorism diffuses across countries via migration flows as a vehicle. We contribute to this debate by studying the influence of migrants’ cultural proximity to the native population of their host country. It is argued that cultural closeness can contain such terrorism diffusion. Similarities in societal norms, customs, or beliefs seem likely to induce trust in the social interactions between migrants and locals. This, in turn, makes it more difficult for terrorist organizations to exploit transnational population movements for radicalization and as a recruitment pool – one of the core mechanisms linking population flows with terrorism. Conversely, migrants from culturally distant societies may find it more challenging to integrate into their new homes. A fertile ground for terrorist organizations for the recruitment of new followers is thereby more likely. Our analyses present consistent evidence that the effect of terrorism diffusing across countries weakens when accounting for cultural closeness between migrants and host societies. This key finding of our research has crucial implications for policy’s and scholars’ understanding of terrorism, the diffusion of terrorism across countries, and the security consequences of population movements.