<p>Sea water desalination has become an essential strategy for addressing chronic water scarcity in arid regions, including Iran. As part of this approach, the Iranian government has proposed the Caspian Sea Water Transfer Project, which combines large-scale desalination with inter-basin water transfer to the country’s central plateau. While this project aims to alleviate domestic water shortages, it raises significant legal and environmental challenges that are not adequately addressed by existing international or regional regimes. The Caspian Sea is governed by a <i>sui generis</i> legal framework, established by the 2018 Convention on the Legal Status of the Caspian Sea and the Tehran Convention, which define it as neither a conventional sea nor a freshwater lake. This unique status complicates the application of traditional principles of international water law and leaves regulatory gaps regarding the environmental impacts of desalination, including brine discharge, chemical pollution, and ecosystem disruption. This article asks how core principles of international environmental law ‒ equitable and reasonable utilization, the no-harm principle, and the duty to cooperate ‒ can be applied or adapted to regulate desalination activities and prevent transboundary harm in the Caspian Sea. It systematically analyzes relevant treaty obligations, customary international law, and regional agreements to identify normative gaps and assess their adequacy in governing desalination-related risks. By clarifying the interaction between regional conventions and general principles of international law, the article proposes a framework for strengthening environmental governance and minimizing potential disputes among the Caspian littoral states.</p>