Climate change is predicted to lead to an increasing frequency of natural disasters and humanitarian emergencies, yet scholars have not examined how the humanitarian community is responding to this issue. This article examines its initial engagement with the climate change regime and finds it was remarkably coordinated. Humanitarian agencies coauthored submissions to the UN Framework Convention on Climate Change, and the leaders of major humanitarian organizations spoke on co-organized panels on the humanitarian perils of climate change. In fact, the overarching trend was cooperation, not competition, among humanitarian agencies. This is an intriguing finding as it runs counter to the dominant account of a humanitarian marketplace in which actors are constantly competing for resources. Instead, this article suggests that the Inter-Agency Standing Committee played a significant role in mobilizing and coordinating humanitarian organizations’ initial efforts. It highlights how and to what extent institutionalized cooperation between international organizations enables further cooperation in new issue areas and regimes. Scholars of international organizations, global environmental politics, and humanitarianism will be interested in how cooperation emerged in the humanitarian regime and shaped subsequent interaction with the climate change regime.