<p>Mountains, often called the world’s ‘water towers’, are vital sources of freshwater that sustain ecosystems and societies far downstream. Climate change is altering their role, impacting the quantity, timing and quality of mountain water supply. While substantial research focuses on snow and glacier changes within mountains, a holistic understanding of how water cycle changes in mountains impact human water use and ecosystems downstream is still needed. Here we address how these changes cascade downstream, affect diverse social–ecological systems and pose challenges for adaptation. We also highlight major research gaps and outline research priorities for improving understanding of these coupled systems under a changing climate.</p>