Scientific experts have become structuring actors in global environmental governance. They have put environmental problems on the agenda, facilitated the establishment of multilateral agreements, and continue to provide the information to support international policymaking. This paper, drawing on the literature on transnational professionals, introduces the notion of ‘transnational diplomat-scientists’ to describe a group of influential scientific experts that has risen to considerable power and increasingly acts as diplomatic actors in environmental agreement-making processes. Through qualitative methods and visual network analysis, we explore the case of the ipcc Bureau members and discuss the professional profiles and multipositionality of its members for the sixth assessment cycle (2015–2023). Strategically positioned at the science-policy interface, these actors connect different disciplines and stakeholders at different levels. Through such a positioning and their capacity to manage expectations and facilitate compromises, diplomat-scientists play a crucial role in upholding the authority of science in multilateral negotiations.