Since the 1980s, the United Nations Environment Programme (UNEP) has been addressing the links between the environment and security, progressively questioning the security implications of climate change. Relying on its scientific expertise and partnerships with other international organizations (IOs) and think tanks, UNEP plays a critical role in raising awareness on climate security and in formulating potential institutional responses. Drawing on the sociology of IOs, this chapter retraces UNEP’s action in the field of climate security, especially looking at its boundary work between climate experts and policy-makers. Relying on data generated through online observation and qualitative analysis of UNEP’s publications, it sheds light on the functional and practical rationality which supports UNEP’s mandate expansion and institutional positioning in this emerging field. Based on this case, the chapter contributes to a better understanding of the role of environmental IOs in framing and responding to the security implications of climate change.