Often supported by influential agricultural unions, recent farmers’ protests across Europe have opposed environmental policies aimed at reducing carbon emissions and improving biodiversity. Despite such strained relationships, exacerbated by climate change mitigation and adaptation measures, some movements unite agrarian and environmental mobilisation. Combining insights from critical agrarian studies and environmental labour studies into a novel relational and historical framework, this article aims to explain such agroecological convergence in the collective action against water reservoirs in western France since 2017. This case, which drew international attention during clashes in Sainte-Soline in March 2023, exemplifies successful cross-movement organising among diverse actors: an agricultural union, a local citizens’ initiative, and a radical environmental collective. Through a qualitative analysis of movement actor publications, the exploratory case study identifies three key conditions that facilitated the alignment: the failure of corporatist governance to resolve water conflicts, a historical mobilisation around peasant labour in antagonism to industrial agriculture, and a dense informal network of interaction. Together, these factors highlight the relevance of localised trajectories of contestation and agrarian change in fostering agrarian-environmental convergence against a large-scale climate change adaptation project. The analysis thus not only sheds light on an underexplored case but also helps conceptualise agroecological coalition-building beyond global opportunity structures and strategic calculations of movement actors.