The objective of this thesis is to investigate how the absence of effective international institutions creates obstacles to the protection of the marine environment from pollution by synthetic polymers in international waters, between the adoption of the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter in 1972 and the adoption of the High Seas Treaty in 2023. It is argued that the lack of accountability among international system actors is related to the absence of effective institutions with a high level of obligation, precision, and delegation to protect areas beyond national jurisdiction; that propose preventive, mitigative, and waste-removal measures and/or drive behavioral changes among states and other involved actors; and that are capable of sustaining effective management of common resources over time. Additionally, the lack of effective accountability mechanisms is linked to the interests of states, which are largely focused on economic goals and growth, as well as the interests of other related actors, such as large corporations, the plastic industry, and NGOs. This study adopts a qualitative approach, using bibliographic research, document analysis, and process tracing through case studies. The main findings indicate that plastic waste pollution in the oceans significantly impacts the environment, the economy, and various dimensions of human and social life. Furthermore, international instruments are shown to be ineffective in addressing pollution caused by these materials, especially in international waters. Finally, the differing interests of the involved actors often create divergent priorities, making cooperation, policy formulation, and the implementation of effective measures difficult. It is concluded that a shift in perspective is essential, recognizing the ocean as a global public good rather than a common good, in order to ensure its preservation and sustainable management.