<p>While the significance of transboundary rivers in contributing to marine and freshwater plastic pollution is broadly recognised, the potential for international water law in addressing this issue has yet to be sufficiently explored. This research paper aims to bridge this gap by contextualising international water law within the broader landscape of transboundary plastic governance, thereby contributing to the development of a comprehensive legal framework for managing plastic pollution. The study conducts a legal analysis of key international legal instruments, particularly the UN Watercourses Convention and the Water Convention, to examine their relevance, applicability, and effectiveness in regulating plastic pollution in transnational river systems. The findings of this study reveal significant synergies between these conventions, highlighting essential components necessary for effective regulation of transboundary riverine plastic pollution. Two key insights of legal and policy significance emerge: first, the imperative to transition from a solely marine-centric framework to a more comprehensive, integrated freshwater-marine governance approach; and second, the critical importance of recognizing geographic interconnectedness and promoting regional cooperation within legal and policy frameworks. The study concludes that prioritising regulatory strategies for transnational river systems, coupled with regional collaboration and geographic integration, is vital for strengthening the effectiveness and adaptability of the anticipated global plastics treaty. This research thus makes a timely contribution to international water law discussions on regulating transnational river plastic pollution amid ongoing global plastics treaty negotiations, providing a foundational legal analysis for stakeholders to develop a more comprehensive framework.</p>