Can transparency enhance the legitimacy of international institutions? As transparency has become a widely applied procedural standard in international politics, a range of institutions have implemented transparency reforms under the presumption that increased transparency can elicit support among relevant audiences. This paper evaluates whether increased transparency in the UN Security Council leads to enhanced legitimacy perceptions among UN member states. The paper first traces the history of Security Council reform since 1990 and draws on interviews with diplomats and observers to describe a transparency reform the Council enacted in 2006. Next, the paper uses longitudinal content analysis to empirically probe the legitimation effects of that transparency reform. The empirical analysis is based on an original dataset of 4303 legitimacy statements made by UN member states in annual UN General Assembly debates over the periods 1990-2006 and 2006-2018. The findings cast doubt over the potential of transparency reform to improve the Council’s legitimacy; and instead suggest that increasing the direct participation of the wider UN membership may be a more viable legitimation strategy. This article contributes to existing international legitimacy literature by providing empirical evidence on the relationship between transparency and legitimacy; and by demonstrating which institutional features that affect the perceived legitimacy of the Security Council.