The Euphrates–Tigris river system is among the most intensely developed freshwater resources worldwide. At the centre of its development are large‐scale dams supplying extensive irrigation schemes and hydroelectric power plants, which substantially impact the river system's dynamic and water quality. Due to the international dimension of the utilization of cross‐border freshwater systems, the impacts of the freshwater developments of Iran, Iraq, Syria and Turkey can only be effectively addressed by environmental impact assessment (EIA), which takes into account transboundary effects. Since domestic EIA regulations of the Euphrates–Tigris riparian States pay little attention to transboundary effects, and no treaty would commit them to transboundary EIA in respect to the river system, such a requirement may only derive from customary international law. This article will show to what extent transboundary EIA is required by customary international law and how far these provisions bind the Euphrates–Tigris riparian States.