This paper proposes the establishment of an “Intergovernmental Science-Policy Platform for Water Sustainability (ISPWAS)” to undertake a science-based global water assessment to forge a new relationship between science and policy-making. Thus it would bring evidence-based knowledge and rigorous analysis to policy-making sustainably addressing increasingly complex interconnected water challenges across sectors and bridge the growing science-policy gap that exists in many countries. ISPWAS would share world-class, science-based, solution-oriented knowledge to support Member States in their decision and innovative policy-making that underpins and addresses the complexities of sustainably managing water resources. It would help to identify and address the severe deficiencies in human capacity and water governance that hinder effective water management. ISPWAS would undertake the global assessment implemented through national entities with an intergovernmental validation mechanism. Governments need to base their policy and decision-making on sound scientific evidence. The widening science-policy gap must be closed to ensure that policies and regulations are more relevant and based on state-of-the-art science. ISPWAS would assist Member States in improving their knowledge of available tools and information for tackling water issues and their use in policy-making. It would facilitate coordination and exchanges among water-related efforts within and between countries and aid Member States in monitoring national or global initiatives. Finally, ISPWAS would help to position water issues at the forefront of global sustainability agendas and processes. In short, it would significantly transform water management and potentially be a major “game-changer” for accelerating implementation of Agenda 2030 and beyond.