In environmental politics, social movements play a crucial role, promoting participatory rights and confronting injustice, inequality, and the interests of the powerful. This article examines an underexplored topic in the literature on social movements, especially in Latin America: the use of litigation to force decisionmakers to comply with participatory formats, specifically in the course of opposition to hydroelectric dams. These projects often are destructive to the local environment and communities. This study examines four cases of environmental litigation that halted dam construction in Brazil and Chile, singling out causal pathways for successful collective action. It focuses on two dimensions of movement success: the implementation of participatory formats and the resulting cancellation of dam projects. In line with the joint effect model of social movement theory, the crosscase comparison of legal disputes shows that pursuing legal strategies in parallel to broad social mobilization and the support of institutional allies, can lead to successful outcomes.