<p>Marine plastic pollution is a growing environmental challenge that requires coordinated policy responses across multiple levels of governance. While international negotiations, including those for a Global Plastics Treaty, seek to establish shared commitments, implementation depends largely on national and subnational action. This study compares how plastic pollution is governed in Australia and Brazil, two federated systems facing similar pressures, but operating under different institutional arrangements. We examine how plastic pollution policies are developed, coordinated, and implemented across various levels of government in selected jurisdictions in Australia and Brazil. Our focus is on the interactions between federal, state, and municipal actors. All jurisdictions examined use a mix of governance approaches, including command-and-control, economic instruments, and enabling strategies, with different emphases shaped by institutional capacity and coordination. These differences affect how effectively plastic pollution policies are translated into action across levels of government. Building on these findings, the study proposes six strategic priorities for strengthening multilevel governance: (i) improving coordination, (ii) clarifying roles, (iii) supporting local implementation, (iv) strengthening data systems, (v) promoting inclusive governance, and (vi) leveraging national policy frameworks. These insights offer a grounded understanding of institutional dynamics in federated systems and may inform efforts to implement the Global Plastics Treaty at national and subnational levels.</p>