Summer mountain pastures are complex coupled ecological and human systems. They provide vital forage for livestock during summer, and their traditional use is decisive for the maintenance of biodiversity, ecosystem services, and open landscapes, which benefit local populations and tourists. With climate change, the increased intensity and frequency of climatic hazards threaten the sustainable management of these systems. To foster climate adaptation in such complex systems, we developed a tool to assess their climate change-related vulnerability. The tool consists of a 3-step vulnerability analysis: first, of the inherent exposure of mountain pastures to climatic hazards based on their physical features; second, of vegetation sensitivity to climatic hazards and changes in practices; and third, of adaptive capacities that lie in the options for managing mountain pastures together with the farms using them. This work was carried out within the research and development network Sentinel Mountain Pastures, which addresses climate change adaptation issues on mountain pastures across the French Alps. We used a transdisciplinary approach that included participatory work with experts and interviews with stakeholders. We believe this diagnostic tool has high potential for practical application to support adaptation on summer mountain pastures, by allowing a shared integrative understanding of the complexity of mountain pasture systems by stakeholders. We hope this will provide new information for policymaking that enhances the resilience of summer mountain pasture systems.