The South China Sea (SCS), the largest semi-enclosed marginal sea in the western Pacific, exhibits complex ecological and hydrodynamic processes that drive significant spatiotemporal variations in its phytoplankton. Based on multi‑year remote sensing data, we investigated the spatiotemporal variations of phytoplankton including biomass and functional group structure in the SCS. Chlorophyll‑a (Chla) concentrations were generally higher in coastal areas than that in the open ocean, with prominent high‑value bands occurring in regions strongly influenced by coastal runoff and mesoscale dynamic processes. Phytoplankton in nearshore waters were generally dominated by Green algae and Diatoms, while offshore waters were dominated by Prochlorococcus and Prokaryote. Under the influence of complex dynamic environments, the seasonal variation patterns of phytoplankton showed marked spatial heterogeneity. This study further investigated the interannual variability of phytoplankton and its environmental drivers in the summertime upwelling region off the western SCS. Results revealed that the interannual variations in Chla concentrations and dominant phytoplankton functional groups were more strongly correlated with the Western Pacific Subtropical High (WPSH) index than with the ENSO index. The WPSH weakened the southwest monsoon through its westward extension and intensification, thereby suppressing upwelling and nutrient supply and consequently reducing Chla concentrations. Based on the analysis of simulation results under the high‑emission scenario SSP5‑8.5 from the CMIP6 models, the WPSH was projected to show an overall intensifying trend in the future, suggesting that Chla concentration in the western SCS might decline in the future.