The participation of non-state actors in multilateral institutions is often portrayed as one way of decreasing the perceived legitimacy deficit in global governance. The literature on non-state actors has identified several ways in which these actors can enhance the legitimacy of intergovernmental organisations and global governance arrangements. Three partially competing normative arguments, or rationales, for the inclusion of non-state actors in international policymaking — functionalism, neocorporatism and democratic pluralism — have been identified. Whereas functionalism highlights the contribution of non-state actors to output legitimacy in terms of expertise, neocorporatism emphasises the inclusion of affected interests, and democratic pluralism claims that non-state actors increase input legitimacy through procedural values. These three normative arguments thus offer different understandings of the motives for the inclusion and representation of non-state actors in international negotiations and diplomacy. Through a single case study of United Nations climate diplomacy, we analyse the extent to which the three rationales for non-state actor inclusion are found in views held by state and non-state actors participating in the annual United Nations climate change conferences. Our results show that different actor groups place varying degrees of emphasis on the different rationales for non-state actor inclusion, even though the neocorporatist rationale remains most favoured overall. We discuss the implications of our findings for the democratic legitimacy of increasing participation of non-state actors in intergovernmental affairs and recent trends in the participation of non-state actors in the international climate change policymaking process.