The Routledge Handbook of Water Diplomacy is a comprehensive guide to understanding and practicing water diplomacy – a framework for building relationships, negotiating shared interests, and managing complex water challenges across physical, political, and societal boundaries. In an era marked by rising scarcity, deepening uncertainty, and growing geopolitical tension, this timely volume offers actionable insights for negotiated problem-solving grounded in both scientific understanding and diplomatic skill. Moving beyond abstract theory and technical fixes, the Handbook introduces a dual-pathway structure designed to meet the diverse needs of its users. The “Working Together” pathway invites readers to engage with water diplomacy through the lens of their roles, whether as professionals, decision-makers, funders, researchers, or affected communities. The “What Matters and Why” pathway highlights key thematic dimensions, including process design, adaptive learning, trust-building, divergent worldviews, and the management of uncertainty. Together, these pathways guide readers through a wide range of case studies, from transboundary river basins to subnational disputes and community-scale water systems, demonstrating how water diplomacy can resolve conflict, enable cooperation, and support adaptive, context-sensitive 'learning by doing' under conditions of complexity and change. Whether addressing a transboundary dispute or a local allocation challenge, this book provides guiding principles, practical tools, and real-world cases to support water solutions that are scientifically credible, socially inclusive, and politically feasible. The Routledge Handbook of Water Diplomacy serves as an essential reference volume for students and scholars of water diplomacy, water governance and water resource management, as well as for policymakers and water professionals who are seeking actionable insights into the nuanced challenges they encounter as they work to promote a more sustainable and equitable water future. The Open Access version of this book, available at http://www.taylorfrancis.com, has been made available under a Creative Commons Attribution- NonCommercial- ShareAlike 4.0 International (CC BY- NC- SA) license.