Characterizing the vertical distribution of chlorophyll a in the water column in each oceanic region is crucial for accurate assessment of depth-integrated phytoplankton biomass. In this study, the characteristics of the subsurface chlorophyll maximum (SCM) in the South China Sea (SCS) during the boreal summer were investigated by using bio-optical and hydrological data collected during four cruises from 2008 to 2015. During the boreal summer, a well-developed SCM layer was found to be a prominent feature in the SCS, with the thickness, depth, and magnitude of the SCM exhibiting large spatial variability. The vertical position of the SCM varied between 11 and 99 m, with an average value of 53 m. Light attenuation played a fundamental role in determining the depth (Z_SCM) and magnitude of the SCM (Chl_max), as reflected by their relationships with the euphotic depth (Z_eu). However, because significant positive correlations were found between Z_SCM and the depth of potential density at 23 kg m⁻³ (Z_σ=23), physical processes were inferred to be more important in modulating the fluctuation of Z_SCM, especially in open-ocean areas. Anticyclonic eddies, which act to deepen the nutricline by means of isopyncnal displacement, may play a role in aggravating nutrient limitation in the SCM layer, which leads to deepening of Z_SCM, weakening of Chl_max, and a fall in the total integrated chlorophyll a within the euphotic layer (Chl_intZeu). Knowledge of ocean physical conditions and the variability of Z_SCM should be taken into consideration to improve the accuracy of Chl_intZeu estimates based on the surface chlorophyll a concentration (Chl_surf), especially when Chl_surf is very low. Our assessment of SCM parameters provides the basis for a better understanding and quantification of their role in primary production estimation within the SCS.