The operations of large hydropower dams in the Mekong basin, while posing detrimental transboundary effects downstream, are not fully recognised in Vietnamese government policies, especially at the regional scale. This study adopts the concept of institutional fit to investigate policy gaps in addressing the transboundary water challenges characterised by falling floodwater flows and accelerating saltwater intrusion in the Vietnamese Mekong Delta. Based on interviews with key stakeholders (including central and provincial government officials, environmental experts, and academics) as well as desk reviews of policy documents, we argue that, while the transboundary water repercussions are increasingly evident on the ground, they are not adequately factored in water policies and strategic development planning for the delta. This study suggests that failure to incorporate co-evolving effects of climate change and Mekongwide hydropower dynamics into local water governance will cause institutional constraints to tackle water stressors (too much or too little water) over the long term.Achieving institutional fit will therefore help align the national water policy framework with local development strategies and reduce policy-practice gaps at the delta scale.To this end, two key aspects need to be considered: first, how, and in which forms, innovative management decisions and water governance policies can be effectively undertaken towards securing the living delta and its resilience to present and future environmental risks; and second, how reframed water policy and development agendas can holistically capture and resolve the transboundary hydrological problems, while leveraging the Vietnamese government's policy engagement at higher levels to address related impacts.