Expert organizations in sustainability governance increasingly adopt participatory strategies to strengthen their knowledge claims. We introduce the notion of knowledge platforms for sustainability to conceptualize expert organizations that not only rhetorically embrace but also actively attempt to institutionalize the norm of stakeholder participation. In seeking authority in sustainability governance, knowledge platforms for sustainability encounter a tension between the ambition of stakeholder participation and principles of scientific autonomy and consensus that have long been perceived as foundational to the epistemic authority of expert organizations. Taking this tension as a starting point, we utilize a dynamic perspective on epistemic authority to investigate the contestations over institutional designs. We compare the institutionalization of participatory designs over time in two knowledge platforms for sustainability – the Intergovernmental Platform for Biodiversity and Ecosystem Services (IPBES) and Future Earth. Our comparison reveals that institutional designs for participation open up the process of knowledge creation and evaluation and allow for the inclusion of a broader knowledge base and set of experiences. Yet, in seeking epistemic authority, knowledge platforms also reinforce existing power structures by redrawing boundaries that protect scientific autonomy and privilege relationships with elite actors. We conclude that the institutionalization of participation is shaped by the way in which knowledge platforms seek epistemic authority (specifically from whom), which in turn shapes whose and which knowledge is presented as legitimate in global environmental politics.