The coastal ocean, comprising rivers, estuaries, tidal wetlands, and the continental shelf, plays a crucial role in the global carbon cycle. Recent evidence suggests that the coastal ocean has become a net sink for atmospheric CO2, with significant implications for its future role in the global carbon budget. The carbon cycle in these coastal systems is influenced by various factors, including climate, land use, and human activities. Riverine inputs of organic and inorganic carbon are substantial, and their regulation by factors such as precipitation, temperature, and hydrologic events is critical. Estuaries act as transitional zones, modulating the exchange of carbon between land and the ocean, with significant losses of organic carbon through microbial degradation and photochemical oxidation. Continental shelves, though covering only 7-10% of the global ocean area, contribute significantly to primary production, carbon burial, and the supply of organic carbon to the deep ocean. Human activities, such as land-use changes, nutrient inputs, and climate warming, have altered the carbon cycle in these systems, leading to increased CO2 uptake and changes in carbon fluxes. The coastal ocean's role as a carbon sink is expected to continue, but the magnitude and timing of these changes remain uncertain due to the complexity and heterogeneity of coastal systems. Future research should focus on improving the understanding of carbon dynamics in these systems and integrating coastal carbon processes into Earth system models to support policy and mitigation strategies.The coastal ocean, comprising rivers, estuaries, tidal wetlands, and the continental shelf, plays a crucial role in the global carbon cycle. Recent evidence suggests that the coastal ocean has become a net sink for atmospheric CO2, with significant implications for its future role in the global carbon budget. The carbon cycle in these coastal systems is influenced by various factors, including climate, land use, and human activities. Riverine inputs of organic and inorganic carbon are substantial, and their regulation by factors such as precipitation, temperature, and hydrologic events is critical. Estuaries act as transitional zones, modulating the exchange of carbon between land and the ocean, with significant losses of organic carbon through microbial degradation and photochemical oxidation. Continental shelves, though covering only 7-10% of the global ocean area, contribute significantly to primary production, carbon burial, and the supply of organic carbon to the deep ocean. Human activities, such as land-use changes, nutrient inputs, and climate warming, have altered the carbon cycle in these systems, leading to increased CO2 uptake and changes in carbon fluxes. The coastal ocean's role as a carbon sink is expected to continue, but the magnitude and timing of these changes remain uncertain due to the complexity and heterogeneity of coastal systems. Future research should focus on improving the understanding of carbon dynamics in these systems and integrating coastal carbon processes into Earth system models to support policy and mitigation strategies.