AbstractThe transport of carbon from land to ocean, via rivers, groundwater, and aerosols, is an important component of the global carbon cycle that must be known to accurately assess anthropogenic CO₂ storage on land and in the ocean. Current global carbon cycle budgets have adopted terrestrial carbon inputs to the ocean ranging from 0.5 GtC/yr to 0.9 GtC/yr, derived mainly from estimates of riverine fluxes. However, these budgets ignore the terrestrial carbon inputs from coastal ecosystems and through submarine groundwater discharge (collectively referred to as coastal margin inputs in this study) due to difficulties in making global assessments. Using a numerical model and globally distributed ocean observations of stable carbon isotopes, we estimate terrestrial carbon inputs to the ocean at 1.4 ± 0.5 GtC/yr, with 95% of the coastal margin inputs (0.8 ± 0.5 GtC/yr) occurring in the Pacific and Indian Oceans. This terrestrial carbon flux is largely balanced by an efflux to the atmosphere of 1.2 ± 0.5 GtC/yr, 40% of which occurs in poorly monitored coastal regions and may have been overlooked in previous observation‐based global estimates. Our results suggest more dynamic cycling of carbon in the land‐ocean transition zone than previously thought, and that rivers may not be the only important pathway for terrestrial carbon to the ocean.