Abstract: Diatom samples were collected from surface sediments in Daya Bay, Shenzhen, Guangdong, South China in August 2021 (summer) and January 2022 (winter) and the seasonal distribution of diatom was analyzed. Distinct seasonal differences in diatom assemblages were observed between the two seasons, and their relationships with local marine environmental changes were explored. Results show that the average abundance of sediment diatoms in summer (2.36×10⁶ valves/g) was slightly higher than in winter (2.12×10⁶ valves/g). Diatom abundance in summer gradually increased from the bay head to the bay mouth, while it decreased from the bay head to the bay mouth in winter. Further analysis revealed that the seasonal changes in water masses and circulation structure driven by the East Asian monsoon were the primary factors responsible for the seasonal differences in diatom abundance and distribution patterns in Daya Bay. Regarding diatom species composition, the spatial distribution of planktonic diatoms in the surface sediments showed a strong correlation with seasonal changes in water temperature, while benthic diatom distributions were more closely related to water depth and hydrodynamic conditions. Therefore, the seasonal differences in water temperature caused seasonal variations in the distribution of planktonic diatoms in the surface sediments in Daya Bay, whereas benthic diatoms exhibited relatively minor seasonal distributional changes. In addition, in coastal areas with high productivity and high sedimentation rates, surface sediment diatom communities may reflect short-term (even seasonal) environmental conditions rather than long-term accumulation. High-resolution sedimentary diatom records from productive coastal regions with relatively stable hydrographic conditions have potential to reconstruct high-temporal-resolution environmental changes, including seasonal-scale variations.