This contribution analyses the proliferation of parliamentary institutions in regional economic organizations around the world. It argues that this phenomenon has not resulted from the spread of structural conditions that were similar to those obtaining in the early European Union – the pioneer of regional parliamentarization. Instead, the proliferation of regional parliamentary institutions was the outcome of a process of norm diffusion that was rooted in the European Union’s active and passive parliamentary entrepreneurship as well as a global participatory discourse. Thus, parliamentarization in the European Union and elsewhere is an interdependent, not independent, phenomenon. After developing these arguments theoretically, the contribution presents quantitative evidence and a case study of parliamentarization in the Association of Southeast Asian Nations (ASEAN) to substantiate these claims.