For medical humanitarian organizations, making their sources of legitimacy explicit is a useful exercise, in response to: misperceptions, concerns over the 'humanitarian space', controversies about specific humanitarian actions, challenges about resources allocation and moral suffering among humanitarian workers. This is also a difficult exercise, where normative criteria such as international law or humanitarian principles are often misrepresented as primary sources of legitimacy. This essay first argues for a morally principled definition of humanitarian medicine, based on the selfless intention of individual humanitarian actors. Taking Médecins Sans Frontières (MSF) as a case in point, a common source of moral legitimacy for medical humanitarian organizations is their cosmopolitan appeal to distributive justice and collective responsibility. More informally, their legitimacy is grounded in the rightfulness of specific actions and choices. This implies a constant commitment to publicity and accountability. Legitimacy is also generated by tangible support from the public to individual organizations, by commitments to professional integrity, and by academic alliances to support evidence-based practice and operational research.