The rapid expansion of the public discussion and research on just transition implies the risk of watering down either justice or the (eco-)socio-technical transition itself. We create a theoretical notion of just transition boundaries and propose it to help consider non-negotiable limits to just transition discourse and make sense of negotiations within such limits. Just transition boundaries are comprised of ecological and social boundaries. They determine that just transition-processes must bring societies effectively within the safety thresholds of the two most (...) critical planetary boundaries, climate change and biodiversity loss, and must do that by means and supportive measures that protect vulnerable groups from falling or getting stuck below social minimums in those processes. Boundaries leave room for plural values and visions for realizing transitions and remaining within safe thresholds in community-specific conditions. Context-specific additions to what just transition should cover are possible insofar as they do not contradict or risk just transition boundaries. In addition to justifying and conceptualizing just transition boundaries, we reflect on its implications for policymaking and research. (shrink)