The complexity and dynamics of digital business ecosystems create ambiguity and challenge agility with regard to regulation. As actors, services, and user interfaces are increasingly interconnected and nontransparent within these ecosystems, traditional disciplinary and existing interdisciplinary methods from the realms of information systems and law become insufficient to cope with the extent and pace of change. To address this challenge, this paper follows a design science research approach to develop a novel method for ecosystem-architecture guided regulatory reasoning. This method is intended to be more collaborative, interdisciplinary, and agile to enable regulatory reasoning in the digital age. By enabling the systematic decomposition of ecosystem architectures, our method helps researchers and practitioners gain transparency about ecosystems and their various interfaces, identify legal issues, and derive policy implications to adjust laws. The paper demonstrates the method based on the Google News ecosystem and discusses identified legal issues in terms of the Interstate Media Treaty.