Seagrass meadows play a critical role in climate regulation through carbon sequestration, yet global patterns and drivers remain poorly quantified. Here we present a comprehensive global assessment of seagrass carbon accumulation rates (CARs) and their economic value. Using published field CAR measurements, we trained state-of-the-art machine learning models to map and project CARs across global seagrass habitats. We estimated a global mean CAR of 23.51 ± 11.76 g C m⁻² yr⁻¹, corresponding to an oceanic carbon burial of 47.95 Tg C yr⁻¹. Subtropical and warm-temperate regions showed the highest accumulation rates, with distinct latitudinal peaks at 30-40° in both hemispheres. Our model identified sea surface height as the primary driver, with human pressures (shipping intensity, nitrogen inputs) and marine biodiversity also strongly influencing CARs. Model robustness varied geographically, with highest reliability in well-studied regions. National-scale analysis revealed marked inequality in blue carbon wealth distribution. Five countries (Australia, United States, Italy, France, Spain) account for 46.2% of global seagrass carbon sequestration, while the top 10% of nations control 67.3% of the total economic value, estimated at US$27.24 ± 0.51 billion annually. These findings provide essential baselines for blue carbon policy and highlight the uneven distribution of nature-based climate solutions across nations.