The ecological state of transboundary water of the Dniester River Basin is analyzed in the context of cross-border cooperation. The major objects of economic activity affecting the state of transboundary water have been identified in the Dniester River Basin in Ukraine and Republic of Moldova. It has been established that over the years of transboundary cooperation, a decrease in the average annual concentrations of Suspended solids, Nitrites, Chlorides and Sulfates below the maximum permissible values has been achieved. The ecological state of the Dniester River transboundary water in time and space was investigated using an integral assessment of the state of the Dniester River water environment. It is recommended to assess the transboundary water ecological state using complex indices. An assessment of the ecological reliability of the Dniester River Basin transboundary water in 2020 was carried out. The research results show that the use of transboundary water of the Dniester River Basin is potentially dangerous. Further cooperation between Ukraine and the Republic of Moldova in the issue of reducing wastewater containing harmful substances with a synergistic effect is encouraged.