The ocean's vegetated habitats, in particular seagrasses, mangroves and salt marshes, each capture and store a comparable amount of carbon per year, forming the Earth's blue carbon sinks, the most intense carbon sinks on the planet. Seagrass meadows, characterized by high primary productivity, efficient water column filtration and sediment stability, have a pronounced capacity for carbon sequestration. This is enhanced by low decomposition rates in anaerobic seagrass sediments. The carbon captured by seagrass meadows contributes significantly to the total blue carbon. At a global scale, seagrass ecosystems are carbon sink hot spots and have profound influences on the global carbon cycle. This importance combined with the many other functions of seagrass meadows places them among the most valuable ecosystems in the world. Unfortunately, seagrasses are declining globally at an alarming rate owing to anthropogenic disturbances and climate change, making them also among the most threatened ecosystems on the Earth. The role of coastal systems in carbon sequestration has received far too little attention and thus there are still many uncertainties in evaluating carbon sequestration of global seagrass meadows accurately. To better assess the carbon sequestration of global seagrass ecosystems, a number of scientific issues should be considered with high priorities: 1) more accurate measurements of seagrass coverage at national and global levels; 2) more comprehensive research into species- and location-specific carbon sequestration efficiencies; 3) in-depth exploration of the effects of human disturbance and global climate change on carbon capture and storage by seagrass ecosystems.