Yes / Global water security presents a complex problem for human societies and will become more acute as the impacts of climate change escalate. Water security connects the practical water and sanitation challenges of households to the dynamics of global hydroclimates and ecosystems in the Anthropocene. To ensure the successful deployment of attention and resources, it is necessary to identify the most pressing questions for water research. Here, we present the results of a scoping exercise conducted across the global water sector. More than 400 respondents submitted an excess of 4,000 potential questions. Drawing on expert analysis, we highlight 100 indicative research questions across six thematic domains: water and sanitation for human settlements; water and sanitation safety risk management; water security and scarcity; hydroclimate-ecosystem-Anthropocene dynamics; multi-level governance; and knowledge production. These questions offer an interdisciplinary and multi-scalar framework for guiding the nature and space of water research for the coming decades.