This article analyses the development of the European Union (EU) as a global actor in the area of climate security. Building on this, it explicitly draws on constructivist concepts such as norm entrepreneurship and epistemic communities. To this end, it adopts the framework of epistemic communities, as developed by Peter Haas, in order to suggest that there is a group of EU officials, EU member states and think-tank activists, who drive the climate security agenda of the EU. Thus, it examines the precise actors involved in this EU epistemic community for climate security. This group promotes a reason for action at the global level, resulting in the attempt to diffuse this norm: climate change has consequences for international security; thus, it requires the development of appropriate policies and capabilities within the EU and globally. This article suggests that the epistemic community on climate security has been effective at diffusing this norm at both levels, albeit with differences.