This article assesses the rationality of the governance of the Vietnamese coastal zone’s water system. We first specify five assessment criteria, which we apply to a case study. Based on document analysis, stakeholder surveys and in-depth interviews, we found an average score on the criterion that relevant water system knowledge must be available. The scores on the criteria that water usage is systematically monitored, that the legal framework is complied with, that long-term human and wider ecological interests are addressed, and that governance is decentralized appeared to be low. The article concludes with some recommendations to change the governance system.