The chapter defines the concepts of the ecosystem approach and the ecological system, which have been established for the first time at the international legislative level. It notes that the state environmental policy of Ukraine is aimed at achieving a good state of the environment by introducing the ecosystem approach to all areas of the country’s socioeconomic development in order to ensure the constitutional right of every citizen of Ukraine to a clean and safe environment, the introduction of balanced nature management, and the preservation and restoration of natural ecosystems, including aquatic ones. The legislation of Ukraine should introduce the ecosystem approach by developing special techniques and methods of legal regulation of relations in terms of protecting and using separate natural ecosystems connected by territory and preserving the ecosystem as a whole. Attention is drawn to the fact that the environmental legislation of Ukraine defines an ecological system and an aquatic ecosystem, but there is no legal definition of the concept for an aquatic ecological system. The science of environmental law describes it as a natural aquatic environment that has a closed system of interrelationships of its components (living and nonliving elements interacting with each other in perfect harmony, connected by metabolism and energy), giving it the stability associated with other sustainable ecosystems, and has a certain biological productivity. Since the quality of the natural environment is maintained as a result of the interaction of all components of nature, environmental legislation should consider the water environment in a complex of interacting ecological systems, in the relationship of its constituent parts, elements, and phenomena. Identifying ecological systems as an independent object of environmental legal relations requires adopting the more detailed provisions aimed at their protection, use, and restoration according to their features.