In the face of the consequences of climate change for vulnerable populations, some humanitarian Non-Governmental Organizations (NGOs) engage in climate advocacy to mobilize decision-makers and public opinion. Others oppose such practices, citing risks of mandate dilution, purpose washing, and ineffectiveness. This thesis therefore asks: must humanitarian NGOs engage in climate advocacy? Nine semi-structured interviews were conducted with professionals from four NGOs (Acted, Action Against Hunger, the French Red Cross, and MSF), from expert bodies (Climate Action Accelerator, Groupe URD), and from Greenpeace. The findings, supported by the literature, highlight the legitimacy of humanitarian actors in addressing climate issues, while also pointing to financial, communicational, ethical, and political trade-offs. Although no legal obligation requires humanitarian NGOs to engage in climate advocacy, a moral duty may justify such action, entailing a politicization of climate issues and raises questions of identity and accountability.