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Riverine organic carbon (OC) transport plays a role in regulating terrestrial and marine carbon pools and deteriorating coastal water quality. However, long-term OC transport in Asian rivers and its diffusion in marginal seas have remained unreported. This study reported the spatiotemporal variations in OC resources for Hong Kong waters, China, based on monthly monitoring data collected at 82 river stations and 94 ocean sites during 1986–2020. The station-based riverine OC varied spatially and was generally high, with a mean value of 1.4–52.0 mg/L. Moreover, along with improving water quality, OC at 97.6% of the river stations decreased during 1986–2020; overall, sewage treatment accounted for 83.4% of the exponential decrease in riverine OC (R 2 = 0.68, p < 0.01). However, the reduction in riverine OC accounted for only 10.4% of the reduction in the marine five-day biochemical oxygen demand (BOD 5), which occurred at 70.2% of the ocean sites, especially those closest to the shore. The linear reduction in the marine BOD 5 (R 2 = 0.24, p < 0.01) was mainly attributed to reduced OC input from the adjacent Pearl River (61.9%) and decreases in phytoplankton growth (19.0%). These results indicated that sewage treatment improved water quality and decreased OC resources in Hong Kong waters, which can serve as a sustainable development model for other coastal cities. This study has important implications for mitigating organic pollution in the context of human efforts to manage the water environment. • Sewage treatment decreased OC resources in waters between 1986 and 2020. • Riverine OC input primarily influenced organic pollution in nearshore waters. • The sustainable management of water resources in Hong Kong is exemplary. [ABSTRACT FROM AUTHOR]