The research deals with the principle of the prohibition of international waterway diversion in the law of international watercourses. The research reviews individual and collective doctrinal efforts that have touched upon the principle as an internationally wrongful act because of its serious damage and consequences for downstream States. The research addresses the nature of the principle of the prohibition of diversion of international watercourses; its various effects; principles of international law establishing the principle of prohibition of diversion; and its application in State practice and international justice. This principle has been enshrined in most international treaties and judicial decisions. The principle of prohibition of diversion is to protect the rights of downstream States by restricting the absolute sovereignty of source States when using the waters of their transboundary rivers. At the end of the study, the violations committed by the riverine States neighboring Iraq (Turkey and Iran) as a result of projects to divert the waterways of the Tigris and Euphrates rivers without consultation and agreement with the Iraqi side, which would have an impact on Iraq's water resources in both quantitative and qualitative terms.