The relevance of research. Transboundary marine ecosystems represent complex natural - economic systems that cannot be effectively governed within the confines of national jurisdiction alone. The Baltic Sea is classified as a semi - enclosed sea and is highly vulnerable to anthropogenic pressures, which makes its protection critically important both for regional environmental security and for the strategic development of coastal territories. Despite the existence of an extensive body of international environmental law, including the 1992 Helsinki Convention and the activities of the Helsinki Commission for the Protection of the Marine Environment of the Baltic Sea (HELCOM), a substantial share of adopted decisions remains recommendatory in nature, while mechanisms of international accountability remain underdeveloped.This reduces the effectiveness of environmental commitments and hinders progress toward sustainable development goals. The purpose of the research is to analyze the international legal and institutional mechanisms governing the protection of the Baltic Sea marine environment and to identify directions fortheir improvement. The objectives of the study are to trace the evolution of international environ mental norms; to assess the role of HELCOM and the effectiveness of the Baltic Sea Action Plan (BSAP); to identify the key institutional limitations of the current governance model; and to determine managerial measures capable of enhancing the effectiveness of international cooperation. The results of the study demonstrate that the existing governance system is characterized by limited legal enforceability and the absence of accountability mechanisms, as evidenced by the fact that only about 8% ofthe measures underthe updated 2021 Action Plan have been fully implemented. The study substantiates the need to expand HELCOM's mandate and to establish a dedicated coordination body that would ensure systematic monitoring of compliance and enhance the effectiveness of international environmental governance.