• We analyze how the urban poor mobilize against floods in the context of a major urban development project in Latin America. • We find a mix of optimism and skepticism regarding the potential changes brought by political and legal mobilization. • The judicialization of flood-related claims in the legal arena coexists with social mobilization in the political arena. • For grassroots movements, litigating against social-ecological injustices is a means to foster better urban governance. By the 1980s, the Brazilian Amazon was already an urbanized forest. A large portion of its population was living in non-rural areas attracted by better service provisioning and economic opportunities in fast-developing urban centers. Located at the Amazon Estuary-Delta, Belém is the Amazon’s largest metropolitan area, marked by informal urban expansion, and serving as an illustrative case of the opportunities and challenges with which urban populations deal in the region. Faced with significant pressures of absent infrastructure, high rates of poverty, precarious settlements, and ever more frequent flood events, already in the 1980s, Belém was home to the biggest urban reform project of its time in Latin America: The Macro-Drainage Project of the Una River Watershed. Costing over 300 million USD, the project was intended to have a multilevel governance system through which the Inter-American Development Bank and local government could coordinate project activities. Yet, due to the combination of poor infrastructure and changing rainfall patterns, about half a million people currently remain vulnerable to flood hazards and property damages. Our analysis uses a socio-legal approach to studying grassroots mobilization against the violation of social and environmental rights aggravated by floods. We also draw on Ostrom’s concepts of action arena and action situation to examine collective action across political and legal arenas within which individuals, grassroots organizations, and the state negotiate/contest flood-related issues. Conducting archival and ethnographic research, we find a story of conflict and cooperation between the civil society and political and legal actors, which accounts for a mix of optimism and skepticism regarding the improvement in the livelihood of the urban poor affected by this developmental program. This interdisciplinary lens contributes to understanding the limits and potentials of collective action to address, whether through political and/or legal mobilization, urban inequalities and social vulnerability to environmental and climate change.