We present a new reconstruction of summer sea-surface salinity (SSS) over the past 15 000 years based on a diatom record from piston core 17940, located on the northern slope of the South China Sea (SCS). The reconstructed diatom-based summer SSS values for the modern period are in accord with instrumental observations of summer SSS in the area. Here, the modern summer SSS is primarily controlled by river runoff, in particular from the Pearl River. The reconstruction presented in this study shows that the summer SSS varied between 33.3 and 34.2 psu over the past 15 000 years. The long-term summer SSS trend closely followed the trend of the orbitally controlled solar insolation at 20°N, suggesting that orbital forcing was the dominant driver of changes in summer SSS in this area. Comparisons to speleothem δ18O data and studies of surface hydrography in the region suggest that changes in solar insolation affected the summer SSS through changes in the East Asian Monsoon and sea-level changes associated with the last deglaciation. Univariate spectral analyses indicate that centennial-scale oscillatory variations in summer SSS were superimposed on the long-term trend. During the deglacial period (c. 12 000–9000 cal. a BP), the dominant periodicity was centred around 230–250 years, whereas a ∼350-year oscillation dominated in the period 2200–4500 cal. a BP. The balance of evidence suggests that these centennial-scale changes in summer SSS may have been driven by solar-induced changes in the East Asian Monsoon, but further evidence is needed to firmly establish this relationship.