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The decadal variability of ocean primary productivity (PP) has mainly been studied using relatively short term satellite data, limiting its in‐depth analysis. Here, by removing the influence of the 13C Suess effect, we extract the PP signal on decadal timescales using a record of coral δ13C values from the northern South China Sea (NSCS), where the physical‐biogeochemical conditions are influenced largely by the winter monsoon. Our results show that the PP in the NSCS increased from 1851 to the 1920s and decreased from the 1920s to 2007, and large superimposed decadal‐scale changes were observed. The in‐phase relationship observed between the PP reconstruction and winter monsoon records suggests that the changes in the PP in the NSCS are linked to decadal‐scale changes in the winter monsoon. Considering that the winter monsoon intensity may weaken in the future under global warming, the PP in the NSCS might decrease in the coming decades. Plain Language Summary: Marine phytoplankton contributes roughly half of the global primary production and thus constitutes a vital component of the marine ecosystem that serves as the foundation of the marine food web. Due to the relatively short length of observation records, evaluations of the long‐term changes in ocean primary productivity (PP) have suffered from large uncertainties, which have hampered further discussion of the decadal variability of PP in a long‐term context. To discuss the decadal variability of the PP beyond the range of satellite observations, we present a coral‐based PP reconstruction since 1851 CE from the northern South China Sea (NSCS), where the physical‐biogeochemical conditions are influenced largely by the winter monsoon. Our results show considerable decadal variability of the PP in the NSCS superimposed on a pre‐1920s increase and a post‐1920s decrease. We also found an in‐phase relationship between the decadal variability of the reconstructed PP and the winter monsoon records over the past 150 years, indicating that the PP in the NSCS is linked to changes in the winter monsoon forcing on decadal timescales. Considering that the winter monsoon intensity may weaken in the future under global warming scenarios, the PP in the NSCS might decrease in the coming decades. Key Points: The decadal variability of primary productivity was extracted from a coral δ13C record by removing the influence of the 13C Suess effectThe coral‐based primary productivity reconstruction for the past 150 years showed a pre‐1920s increase and a post‐1920s decreaseThe primary productivity of the northern South China Sea varied in phase with the intensity of the winter monsoon on decadal timescales [ABSTRACT FROM AUTHOR]