The participation of practitioners in transdisciplinary sustainability research has been heralded as a promising tool for producing ‘robust’ knowledge and engendering societal transformations. Although transdisciplinary approaches have been advanced as an effective avenue for generating knowledge positioned to question and transform an unsustainable status quo, the political and power dimensions inherent to such research have hardly been discussed. In this article, we scrutinise the constitution of participation in transdisciplinary research through a power lens. Guided by social theories of power and a relational understanding of participation, we analyse how diverse actors equipped with a variety of material and ideational sources wield power over the subjects, objects, and procedures of participation. We applied a qualitative meta-analysis of five transdisciplinary projects from a major German research funding programme in the field of sustainability to unveil the ways in which the funding body, researchers, and practitioners exercise instrumental, structural, and discursive power over (i) actor selection and (re-)positioning, (ii) agenda setting, and (iii) rule setting. We found that researchers primarily exert instrumental power over these three elements of participation, whereas practitioners as well as the funding body wield primarily structural and discursive power. By elucidating tacit and hidden power dynamics shaping participation in transdisciplinary research, this article provides a basis for improving process design and implementation as well as developing targeted funding instruments. The conclusions also provide insights into barriers of participatory agenda setting in research practice and governance.