The concept of megaregions is increasingly put forward among academics and policy makers as a new scale of economic co-ordination and social organisation. A megaregion is most commonly understood as an economic unit that comprises an agglomeration of cities and its less dense hinterlands, which are linked through infrastructure, economic connections, settlement patterns and land use, topography, an environmental system or a shared culture and history that together shape a common interest for the wider region. While there is an extensive literature on the potential benefits of a megaregion, work has been more limited in terms of identifying megaregions in an international context. This paper introduces an approach to delineate potential megaregions in the OECD.