Photosynthetically Available Radiation (PAR) is the portion of solar radiation that supports photosynthesis, and it plays a critical role in marine primary productivity and ecosystem dynamics. This study investigates the spatial and temporal variations of PAR in the East China Sea over a 21-year period (2003–2023) and its implications for marine ecosystems. Satellite-derived PAR data over the East China Sea are analyzed using empirical orthogonal function (EOF) and X11 analyses, revealing that seasonal variations are the predominant signal in the PAR distributions. After removing the seasonal component, the non-seasonal EOF mode 1 shows a significant correlation with the El Niño-Southern Oscillation (ENSO), and the correlation coefficient of 0.55 (p < 0.05) suggests a linkage between the interannual variability of PAR in the East China Sea and broader climatic patterns. We then incorporate the sediment and chlorophyll data from models in empirical formulas to derive the vertical attenuation of PAR throughout the water column and to reveal its seasonal patterns within the water column. The euphotic zone depths calculated from this in-water PAR distribution exhibit a seasonal pattern similar to that obtained from remote sensing data, and they reach their maximum depths in summer due to decreased sediment levels and increased surface PAR. Our results contribute valuable insights into the influence of light availability on marine primary productivity.