Legitimacy is a crucial quality that contributes to the effectiveness and longevity of international institutions. However, over the past years, many institutions have faced increased legitimacy challenges and contestation by member states. This thesis focuses on one key source of frustration for states, namely, the decision-making procedures of international institutions, which are frequently accused of exclusivity, bias, or opacity. It draws on extant literature, both normative and empirical, to develop an analytical framework for understanding procedural legitimacy deficits and the legitimation strategies that can be adopted to address them. More specifically, the thesis defines three types of procedural justice reform—participation, transparency, and impartiality—and hypothesizes that the implementation of these will lead to increases in the legitimacy of international institutions. It then empirically tests whether these three types of reform lead to enhanced legitimacy perceptions among member states of international institutions. To measure the empirical legitimacy effects of procedural justice reforms, this thesis conducts in-depth case studies of three reforms that have been implemented in international institutions: a participation reform in the climate negotiations under the United Nations Framework on Climate Change (UNFCCC); an impartiality reform of dispute settlement in the global trading architecture under the General Agreement on Tariffs and Trade (GATT) and the World Trade Organization (WTO); and a transparency reform in the working methods of the UN Security Council (UNSC). In all three cases, the thesis identifies the legitimacy effects of procedural justice reform by using content analysis to code how states confer and withhold legitimacy in institutional debates and discussions before and after a reform is implemented. The content analysis generates a database consisting of 9546 legitimacy statements made by the memberships of the three institutions over time. These data measure states’ perceptions of legitimacy across three legitimacy categories—what I refer to as procedural, outcome, and diffuse legitimacy. Overall, the three case studies demonstrate that decision-making procedures constitute an important source of states’ perceptions of the legitimacy of international institutions, and that enhancing procedural justice can improve these perceptions. In all three cases, procedural justice reforms significantly affect how states view the legitimacy of international institutions, even when controlled for the effect of institutional outcomes. However, the legitimacy effects are not unequivocally positive. The thesis finds that procedural justice reforms have had positive legitimacy effects in the climate negotiations under the UNFCCC and in the WTO Dispute Settlement System, but negative effects in the UN Security Council. I posit three explanations for the differing effects. First, the reforms re-redistribute procedural control over the institutions to different degrees, leading to differentiated effects: the participation and impartiality reforms lead to positive legitimacy effects for economically weaker states, while the transparency reform leads to positive legitimacy effects for economically stronger states. Second, the reforms generate different degrees of procedural legitimacy trade-offs. Third, the salience of outcomes varies across the three cases. In sum, these different legitimacy effects offer instructive and policy-relevant lessons for how the procedural designs of international institutions influence states’ legitimacy perceptions, as well as for the relationship between different mechanisms of procedural justice.