Abstract Coastal erosion is an increasingly dominant sediment source in marginal seas, particularly in low‐lying areas affected by deltaic subsidence and sediment deficits from upstream water management. However, its role in sediment and organic carbon (OC) dynamics remains to be estimated. Our analyses of the inner Gulf of Thailand (IGoT) revealed that riverine sediment fluxes decreased from 6.6 to 5.4 Mt/yr after 1975, while sediment accumulation within the IGoT increased from 20.8 to 29.5 Mt/yr. The observed trend indicates major sediment contributions from coastal erosion, particularly from mangrove deposits. This process destabilizes coastal ecosystems and accelerates OC decomposition, that is, a low burial efficiency (16.8 ± 5.5%) leads to CO 2 release. Extrapolating these findings globally, mangrove loss could release ∼175 Tg/yr CO 2 . As coastal erosion intensifies under sea‐level rise and human land‐use practices, preserving coastal ecosystems is critical for mitigating blue carbon loss and maintaining coastal stability and resilience. Plain Language Summary Coastal areas are increasingly eroding due to rising sea levels, reduced riverine sediment supply, and deltaic subsidence. The impact on global and regional carbon dynamics remains poorly resolved although the carbon cycle is closely tied to a changing climate and human land‐use practices. In the inner Gulf of Thailand, riverine sediment discharge has decreased from 6.6 to 5.4 Mt/yr since 1975 because of upstream river damming and field irrigation management. However, the total sediment accumulation within the inner gulf is much higher than the river input and has also increased from 20.8 to 29.5 Mt/yr during the same time. This discrepancy indicates that coastal erosion, especially of mangrove deposits, is now a major source of sediment and organic carbon. However, the organic carbon freshly from eroded coastal mangrove deposits decomposes significantly, releasing carbon dioxide into the atmosphere. Based on these findings, global mangrove loss could emit around 175 million tons carbon dioxide per year. As sea level rises and human land‐use practices intensify, protecting coastal ecosystems is crucial to prevent further carbon loss and maintain stable shorelines. Key Points Since 1975, sediment accumulation in the inner Gulf of Thailand (IGoT) increased from 20.8 to 29.5 Mt/yr but riverine fluxes decreased from 6.6 to 5.4 Mt/yr Coastal erosion contributes significantly to IGoT sediment accumulation but the eroded coastal organic carbon has a low burial efficiency (16.8 ± 5.5%) Global mangrove loss could contribute ∼175 Tg/yr CO 2 , making preserving coastal ecosystems critically important