River-bay systems are transitional areas that hold important roles in biogeochemical processes between continents and oceans. However, composition and structure of microbial communities shaped by such environments have not been clear yet. In this study, we used high-throughput sequencing of 16S rRNA genes to analyze the diversity and composition of sediment bacterial communities from the Shenzhen river-bay system during dry and wet seasons. The results showed that sediment bacterial community structure was varied according to habitats (river vs. estuary) and seasons (wet season vs. dry season). The alpha diversity of sediment bacterial community was significantly higher in the dry season than in the wet season, while no significant difference of alpha diversity was found between river and estuary. Neutral community model revealed a significant influence of stochastic processes on sediment bacterial community assembly, especially in the wet season. However, the beta nearest-taxon index indicated that deterministic processes were more responsible for the assembly of sediment bacterial community. Additionally, redundancy analysis suggested strong links between sediment bacterial communities and environmental factors in Shenzhen river-bay system, with the environmental factors explaining 63.5% of the bacterial community variation. Specifically, NH4+, pH, and salinity were the three most important contributing factors that shaped the sediment bacterial communities. Overall, this study provides a valuable reference to get insights into the spatiotemporal pattern of sediment bacterial communities in a typical river-bay system.