Abstract: As a crucial interface between fluvial and marine environments, the prodelta of small- and medium-sized rivers plays a significant role in Earth systems. Imporving research on the sedimentary characteristics of these prodelta systems is essential to better understand the depositional architecture of small- and medium-sized river deltas and to provide scientific guidance for their management. The Jiulong River is a subtropical small- and medium-sized river. Although numerous studies have focused on its subaqueous delta, investigations specifically targeting its prodelta remain relatively scarce. In particular, it remains unclear whether a prodelta exists in the Jiulong River, and if so, its spatial position and extent. This study focuses on the Jiulong Estuary and Xiamen Outer Harbor. By interpreting sub-bottom profiles across the study area, typical deltaic sedimentary characteristics are identified. Combined with comprehensive analyses of sediment provenance, marine sedimentary dynamics, and sediment grain size, these deposits are inferred to be prodelta sediments of the Jiulong River. Dominated by terrigenous fine-grained materials transported by the Jiulong River, these prodelta deposits exhibit a wedge-shaped geometry in shallow seismic profiles, dipping seaward and pinching out toward underlying strata, and they occur below the normal wave base. Spatially, the Jiulong River prodelta extends southeastward from the line connecting Songyu, Jiyu, and Yuzaiwei to the island chain around Qingyu and Wudan Islands, with its distribution jointly controlled by hydrodynamic conditions and seabed topography.