Worldwide, water resources management is increasingly challenged by water scarcity and climate change, which can trigger and aggravate other ecological (e.g. pests) and socioeconomic threats (e.g. commodity price fluctuations) via feedback loops and cascading impacts across systems. In this context, planning for the future is rife with uncertainties for which conventional decision-making methods and policies are inadequate. The objective of TRANSCEND is to identify and catalyze the adoption of Transformational Adaptation Policies (TAP) to water scarcity, including innovative allocation systems and economic instruments, that are robust and adaptable to uncertainty and change, while simultaneously achieving equitable and sustainable economic growth and welfare. To this end, TRANSCEND will develop a groundbreaking ecosystem of innovation that combines three key pillars: (i) a knowledge network for stakeholder engagement and knowledge sharing, (ii) an actionable modeling suite that integrates interdisciplinary socio-ecological science and ensemble forecasting to guide TAP design, and (iii) an accounting and monitoring toolbox that supports implementation and enforcement of TAP in practice. TRANSCEND will apply the ecosystem of innovation to design and implement TAP in 7 labs: Júcar River Basin (RB) (Spain); Reno RB (Italy); Tympaki RB (Greece); Nitra RB (Slovakia); Caplina-Mauri-Desaguadero RB (Peru, Chile & Bolivia); Orontes RB (Lebanon, Syria & Turkey); and Mahanadi RB (Indian states of Chhattisgarh & Odisha). TRANSCEND will leverage this diverse set of demonstrators to initiate adoption of the ecosystem of innovation in 8+ inspiration labs, train 160+ transformation agents, and mainstream uncertainty analysis in key national and European Green Deal strategies. This will provide the knowledge and tools to catalyze robust and systemic transformations to water scarcity and climate change globally, with a clear impact pathway towards TAP adoption in 100+ basins by 2030.