Transnational regulation has become a central part of economic, political and social life, whether it targets ‘blood diamonds’, the depletion of fish stocks in international waters, or labour standards for textile workers. Despite concerns being expressed about the governance, legitimacy and transparency of transnational regulatory regimes, there has not yet been a systematic attempt to develop a large-N study of these regimes. Focusing on the key challenge of transparency, the proposed paper builds on a new dataset of transparency practices of 150 transnational regimes. Addressing the question why some regimes are more transparent than others, we particularly explore the role of stakeholder involvement in the governance of these regimes, controlling for factors such as regime age and capacity, policy sector, and type of regulatory approach. Thus, this study provides a new approach towards the study of and novel insights into the legitimacy of governing arrangements that increasingly affect economies and societies across the globe.