Scholars in sustainability science as well as research funders increasingly recognize that a shift from disciplinary and interdisciplinary science to transdisciplinary (TD) research is required to address ever more complex sustainability challenges. Evidence shows that addressing real-world societal problems can be best achieved through collaborative research where diverse actors contribute different kinds of knowledge. While the potential benefits of TD research are widely recognized, its implementation remains a challenge. In this article, we develop a framework that supports reflection and co-learning. Our approach fosters monitoring of the collaboration processes, helps to assess the progress made and encourages continuous reflection and improvement of the research processes. The TD co-learning framework has four dimensions and 44 criteria. It is based on a substantial literature review and was tested in a Horizon 2020-funded research project ROBUST, which is applying experimental governance techniques to improve rural-urban relations in eleven European regions. The results demonstrate that the framework covers the key facets of TD collaboration and that all four broad dimensions matter. Each research-practice team reflected on how their collaboration is going and what needs to be improved. Indeed, the coordination team was able to see how well TD collaboration is functioning at a project level. We believe the framework will be valuable for actors involved in the planning and implementation of any type of multi-actor, interactive, innovation, transformation and action-oriented research project.