Seagrasses are one of the most valuable coastal and marine ecosystems on the planet, providing a range of critical environmental, economic and social benefits. They provide food and livelihoods to hundreds of millions of people, and they support rich biodiversity, with their sediments constituting one of the planet’s most efficient stores of carbon. However coastal development and population growth, rising pollution and climate change, are threatening the survival of this vital ecosystem. This global synthesis report, which is the first of its kind, aims to improve our understanding of the value of seagrasses and provide recommendations to protect and manage them. One billion people live within 100km of seagrass meadows and 20 per cent of the world largest fisheries depend on these ecosystems. Emissions from loss of seagrass are estimated to contribute up to 299 Tg carbon to the atmosphere per year. At a time of climate emergency, the worrying decline of global seagrass area, estimated to be about 30 per cent since the late nineteenth century, requires a range of actions and policies that recognize the multiple benefits of seagrass ecosystems. Maintaining the health of seagrass ecosystems is important for healthy marine life and for healthy people around the world. In doing so, they represent powerful nature-based solutions to the climate challenge and sustainable development.