Expert organizations 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. In doing so, they encounter a tension between the ambition of stakeholder participation and conventional foundations of epistemic authority, such as scientific autonomy and consensus. 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 participation in two knowledge platforms for sustainability—the Intergovernmental Platform for Biodiversity and Ecosystem Services, and the research platform Future Earth: Research for Global Sustainability. Our comparison reveals that institutional designs for participation open up the process of knowledge creation and evaluation. 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.