he Tigris-Euphrates basin is of significant regional and global importance, and the lack of a comprehensive agreement among the riparian States has turned the utilization of this transboundary watercourse into a political and security concern. The principle of not causing significant harm is a well-established customary and conventional rule in international law, obligating the riparian States. This study aims to clarify the concept of this principle descriptively and analytically, outline its customary and conventional sources, and identify factors influencing its implementation. Additionally, it examines the geographic, natural, and environmental conditions of the Tigris-Euphrates basin and assesses how this principle is applied within the basin. Various development projects, such as constructing excessive dams beyond the river\\\\\\\'s capacity, unsustainable groundwater use, and numerous irrigation canals, exemplify violations of the principle of not causing significant harm, which the riparian States have, to varying degrees, committed. These actions have led to disastrous consequences for agriculture, the environment, and the livelihoods of the basin\\\\\\\'s inhabitants. The urgent need for all basin States to cooperate in developing a comprehensive plan for the equitable and reasonable utilization of the Tigris and Euphrates, avoiding significant harm to other riparian States, and reconciling the principles of equitable and reasonable utilization with the no-harm rule is essential to safeguard the rights and interests of all parties involved and maintain a sustainable regional ecology.

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