In sustainability research, transdisciplinary (TD) approaches that involve practitioners in the research process have emerged as promising tools for enhancing real-world knowledge and engendering societal change. However, empirical insights into how such participation can contribute to the societal effects of TD research are scant and largely rely on single case studies, neglecting practitioners’ perceptions. In this article, we empirically investigate the perceptions of both researchers and practitioners on how practitioners’ participation in TD research might instigate societal changes. We present the results of a qualitative meta-level study of participation processes in seven TD sustainability research projects from a large German research funding programme. Applying a systems approach, we (i) uncover direct, indirect and interlinked participation-effect pathways; and (ii) highlight feedback effects that shape a dynamically evolving participation process. By elucidating both researchers’ and practitioners’ perceptions of participation-effect pathways, this article contributes empirical insights to an emerging scholarship on theories of change in sustainability research and provides ideas on how to better include systems thinking into TD research and future studies of societal effects.