A late Pleistocene delta (Hainan palaeo river delta, HNPD hereafter) exceeding offshore Hainan Island, northwestern South China Sea (SCS), is investigated using high-resolution seismic and core data to understand the relationship between subaqueous delta development and climate drivers during the Last Glacial Period. The seismic data indicate general progradational configurations toward the southwest or southeast, indicating that the sediments from the eastern slope offshore southwestern Hainan Island were transported southwestward and southeastward. The average thickness of the delta sediment is approximately 35 m, and it has an arcuate shape surrounding the eastern slope. Therefore, the sediment provenances of the HNPD were mainly from the Red River drainage and Hainan Island. Comparison between the core dating results and the global sea-level curve indicates that the delta formed mainly during marine isotope stage 4 (65–56 ka). The topography of the basin, the sea-level change from low stand to high stand, and the southward oceanic currents driven by the glacial-period strong winter monsoon all contributed to the formation of the delta. Because the development of the delta required large riverine sediment input, we speculate that the main reason that the delta’s development ceased was the migration of the river channel along the eastern slope. Based on a comparison between the palaeobathymetric scenarios derived from published sea-level curves and the delta stratigraphy identified from the seismic profiles and cores, we have determined a possible range of relative sea level between 65 and 56 ka for the southwestern coast of Hainan Island, which might also be applicable for a broader region, i.e., the northern SCS.