In the traditional view, riverine organic matter typically has a higher C:N ratio than marine phytoplankton 6.7:1 and has therefore been thought to be a carbon source in estuaries and coastal waters. Thus, a decrease in the riverine organic C:N ratio to <6.7:1 would potentially switch riverine organic matter from a coastal carbon source to sink. However, few studies have paid an attention to such a change. Our field investigation showed that organic C:N ratio was 11.8:1 in the pristine upstream section of a natural reserve, but decreased after the river passed through several urban cities, reaching 5.0:1 in near the Pearl River estuary. Along the river, dissolved inorganic nitrogen, total organic carbon and nitrogen all increased and they were highly negatively correlated with organic C:N ratios. The observation has a great implication that organic matter with a decreased C:N ratio from the Pearl River would potentially switch from a coastal carbon source of 2.8 × 1011 g C/year to a sink of 2.2 × 1011 g C/year. This carbon sink (2.2 × 1011 g C/year) contributes to 56% of the previous estimate of the Pearl River estuarine-coastal net carbon sink. Such a decrease in organic C:N ratio also occurs in some other large rivers, which should be considered in the assessment of global coastal carbon budgets and metabolic balance. Display omitted •Organic C:N decreases downstream from 11.8:1 to 5.0:1 along the Pearl River.•The Pearl River organic matter switches from a coastal carbon source to sink.•River organic C:N decrease affects global coastal metabolic balance and carbon budgets.