International parliamentary institutions (IPIs) have become an established feature of international politics. While scholars of international institutions have extensively studied why states delegate to international organizations (IOs) in general, they have said little about the creation of parliamentary bodies. Moreover, IPIs do not fit the functions commonly attributed to international delegation. By differentiating between general-purpose and task-specific IOs, we hypothesize that general-purpose IOs establish and maintain parliamentary bodies that serve their legitimation needs. A nested quantitative and qualitative analysis based on an original dataset on the emergence of IPIs and case studies on the reform of the Economic Community of West African States and the development of the Pacific Islands Forum supports this explanation.