Including scientific and non-scientific knowledge systems in the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) will influence the framing of biodiversity issues and management policies. Through analysis of documents from the IPBES meetings and interviews with non–state actors, this study identifies knowledge systems relevant for biodiversity issues, explores perceptions of why diverse knowledge systems should be incorporated into the IPBES and discusses governance challenges. This study draws upon research in science and technology studies to discuss democratic inclusion of knowledge into the IPBES on the international level. The social development pillar of sustainability underlies the substantive and normative rationales for including diverse knowledge systems into the IPBES. States involved in the IPBES negotiations face the challenge of designing mechanisms that reflect the predominating rationales for including a plurality of knowledge systems in this biodiversity decision–making tool.