The coastal ocean's carbon cycle is a dynamic component of the global carbon budget, but its sources, sinks, and interactions remain poorly understood. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide since the industrial era. Human activities are likely to significantly impact the future evolution of the coastal ocean's carbon budget. The coastal ocean includes several interconnected ecosystems such as rivers, estuaries, tidal wetlands, and the continental shelf. Carbon cycling in these systems is a major component of global carbon cycles. Carbon fluxes within and between coastal subsystems are substantial and are influenced by climate and anthropogenic changes. Understanding these fluxes is essential for accurately accounting for their impact on the ocean and global carbon budgets. Although the coastal contribution to the anthropogenic carbon dioxide budget was previously neglected, it has been recognized recently. The challenge in constraining carbon exchanges in coastal settings is due to the difficulty in scaling up observational studies. However, recent advances in data collection and new biogeochemical contexts have made this an exciting time for the field. New coupled hydrodynamic biogeochemical models can now mechanistically incorporate factors controlling carbon dynamics, such as elemental stoichiometry and biological turnover of organic matter and nutrients. This review discusses the current understanding of organic and inorganic carbon sources, fates, and exchanges in the coastal ocean, emphasizing factors contributing to net carbon fluxes. Carbon inputs and transformations are considered in the context of net air-water CO₂ exchanges, carbon burial in coastal subsystems, and exports to the open ocean. The coastal carbon cycle has fundamentally shifted in recent years due to human activities, making the coastal ocean a net sink for atmospheric CO₂ and a burial site for organic and inorganic carbon. Riverine carbon inputs are important to the steady-state chemistry of the oceans. Riverine fluxes of organic and inorganic carbon continue to be improved by new geospatial tools and scaling approaches. These fluxes are not greatly different from earlier estimates. Average annual carbon fluxes to all major ocean basins and seas are now available. Fluxes are generally well correlated with river discharge, except in certain regions where factors such as high peat and carbonate coverage, and high erosion rates in watersheds also control carbon inputs. Climate is a major driver of river carbon supply to the coastal ocean. Watersheds with high precipitation have higher riverine discharge rates. Temperature also regulates important abiotic and biotic processes that can alter water throughput, flow paths, dissolution rates, and watershed carbon stocks. The net effect of temperature on carbon fluxes can vary between regions and among different forms of carbon. Hydrological events such as extreme rainfall from tropical storms are disproportionately important to riverine organic carbon transport. These storms are responsible for most particulate organic carbon (POC) export from watersheds to the coastal ocean, especially in mountainous regions. Increases in riverine dissolved organic carbon (DOC) concentrations can also result from these events.The coastal ocean's carbon cycle is a dynamic component of the global carbon budget, but its sources, sinks, and interactions remain poorly understood. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide since the industrial era. Human activities are likely to significantly impact the future evolution of the coastal ocean's carbon budget. The coastal ocean includes several interconnected ecosystems such as rivers, estuaries, tidal wetlands, and the continental shelf. Carbon cycling in these systems is a major component of global carbon cycles. Carbon fluxes within and between coastal subsystems are substantial and are influenced by climate and anthropogenic changes. Understanding these fluxes is essential for accurately accounting for their impact on the ocean and global carbon budgets. Although the coastal contribution to the anthropogenic carbon dioxide budget was previously neglected, it has been recognized recently. The challenge in constraining carbon exchanges in coastal settings is due to the difficulty in scaling up observational studies. However, recent advances in data collection and new biogeochemical contexts have made this an exciting time for the field. New coupled hydrodynamic biogeochemical models can now mechanistically incorporate factors controlling carbon dynamics, such as elemental stoichiometry and biological turnover of organic matter and nutrients. This review discusses the current understanding of organic and inorganic carbon sources, fates, and exchanges in the coastal ocean, emphasizing factors contributing to net carbon fluxes. Carbon inputs and transformations are considered in the context of net air-water CO₂ exchanges, carbon burial in coastal subsystems, and exports to the open ocean. The coastal carbon cycle has fundamentally shifted in recent years due to human activities, making the coastal ocean a net sink for atmospheric CO₂ and a burial site for organic and inorganic carbon. Riverine carbon inputs are important to the steady-state chemistry of the oceans. Riverine fluxes of organic and inorganic carbon continue to be improved by new geospatial tools and scaling approaches. These fluxes are not greatly different from earlier estimates. Average annual carbon fluxes to all major ocean basins and seas are now available. Fluxes are generally well correlated with river discharge, except in certain regions where factors such as high peat and carbonate coverage, and high erosion rates in watersheds also control carbon inputs. Climate is a major driver of river carbon supply to the coastal ocean. Watersheds with high precipitation have higher riverine discharge rates. Temperature also regulates important abiotic and biotic processes that can alter water throughput, flow paths, dissolution rates, and watershed carbon stocks. The net effect of temperature on carbon fluxes can vary between regions and among different forms of carbon. Hydrological events such as extreme rainfall from tropical storms are disproportionately important to riverine organic carbon transport. These storms are responsible for most particulate organic carbon (POC) export from watersheds to the coastal ocean, especially in mountainous regions. Increases in riverine dissolved organic carbon (DOC) concentrations can also result from these events.