The concept of agency is increasingly used in the literature on sustainability transitions. In this paper, we add to that discussion by arguing that the concept of rationality opens new avenues to theorizing relational agency in transitions toward a circular economy. To this end, we compare rationality conceptions from management (e.g., collaboration and competition) with critical theory perspectives on rationality (e.g., instrumental and communicative rationality). This leads us to develop a typology matrix for describing plural rationalities underpinning relational agency. We illustrate this typology using excerpts from an in-depth case study of an ongoing city-coordinated ecosystem that develops a smart technology-enabled urban area based on the principles of circularity. The first contribution of this interdisciplinary paper is to offer a rational perspective on theorizing the antecedents of relational agency in circular economy transitions, where communicatively rational action enables agency and change. Secondly, our paper contributes to the literature on circular cities through conceptualizing circular transition as simultaneous collaboration and competition. Thirdly, our paper introduces a dyadic perspective on rationality to the literature on coopetition and provides an operating space from which professionals can navigate, depending on the type of coopetitive situation.