Achieving global water sustainability through a resilient international legal architecture presents one of the most pressing challenges within our resource finite planet. A staggering 42 percent of the total land area of the earth is covered by transboundary river basins, where more than 40 percent of the global population lives and depends on the ecosystem services of the 286 transboundary river basins and 200 transboundary aquifers stretching across the political boundaries of 151 countries. There is already evidence of water resources becoming a source of conflict in many regions and constraining a whole myriad of securities – climate, human, environmental, food, economic, energy – on various levels of society. The international legal architecture to manage this critical natural resource is the overarching area of inquiry in this thesis, and requires improvement to address current and predicted future transboundary water challenges, conflicts and strengthen cooperation. Despite the establishment of around 690 river basin treaties, many of these agreements completely miss or provide unclear provisions on principles and rules of international water law. Until recently there was no legally binding global treaty on transboundary watercourses and customary international law has provided the default rules in the absence of agreements and facilitated the re-interpretation of older agreements in accordance with the Vienna Convention on the Law of Treaties. Now there are potentially two global treaties, with the recent entry into force of the 1997 UN Watercourses Convention and the global opening up of the 1992 pan-regional United Nations Economic Commission for Europe (UNECE) Water Convention, to all UN member states. There is also a plethora of other international environmental legal and non-legally binding instruments, indirectly addressing international law relevant to transboundary watercourses and freshwater ecosystems. Legal regimes for the protection and use of international river basins cannot be interpreted and applied in isolation from other relevant norms of international environmental and general international law. This thesis seeks to understand the rising role and contribution of regional approaches relevant to international law on transboundary watercourses and freshwater ecosystems. More specifically it explores the contribution of the UNECE Water Convention and other relevant UNECE environmental instruments as a structurally distinctive ‘regime’. This thesis introduces a novel conception of a broader ‘UNECE water regime’ which includes the Water Convention, the Convention on Access to Information, Public Participation in Decision-making and Access to Justice (Aarhus Convention), the Convention on Transboundary Environmental Impact Assessment (Espoo Convention), the Convention on the Transboundary Effects of Industrial Accidents, as well as their protocols and non-binding instruments. This research demonstrates how these instruments and their institutions can be interpreted and understood to form a common framework of rules, principles and approaches which fills critical gaps in basin treaties, and collectively contributes to the clarification and development of international law on transboundary watercourses and freshwater ecosystems. This analysis also explores institutional interaction and coordination between and beyond the UNECE pan-regional agreements, as well as the role of soft law or non-binding instruments, and state and non-state actors in the regime. This thesis seeks to contribute to a more coherent understanding of the relationship between the UNECE water regime, international water law, international environmental law and general international law. The UNECE water regime has contributed to clarifying many of the cornerstone rules and principles of international water law and it is argued that the UNECE water regime is lex specialis, which can and mostly does go beyond the UN Watercourses Convention. The UNECE water regime has also arguably spearheaded a paradigm shift in international water law, which sees it moving beyond its historically predominant focus on issues of transboundary impact and utilisation towards a stronger ecosystem orientated approach to environmental protection and equitable use of transboundary river basins. This research identifies key elements of an ecosystem approach, drawing from international environmental and international water law and demonstrates how the ecosystem approach, including ecosystem services, as supported by the UNECE water regime, affects interpretation of international water law towards enhancing ecosystem protection and intra-state equity. This research also explores how the UNECE regime goes beyond what exists elsewhere in international law and international water law on public participation and access to justice. Finally, this research examines the contribution of the UNECE regime vis-à-vis international and European Union water law, across the spectrum of pan-European river basins, especially focusing on the Danube, Sava and Western Bug basins. The UNECE water regime is the most evolved pan-regional regime of its kind, providing ambitious detailed standards and clarification of rules and principles relevant to transboundary watercourses and freshwater ecosystems. It also provides a valuable model of institutional cooperation, progressively engaging state and non-state actors. As this regime takes steps towards realising its global ambition, with almost all instruments now open to all UN member states, and the recent accession by Chad to the Water Convention, this analysis demonstrates why this is predominantly a positive endeavour but also highlights potential challenges and hurdles. This research thus explores the implications and benefits of the UNECE’s rising role in strengthening the international legal architecture to protect the world’s fragile transboundary watercourses and freshwater ecosystems.