Abstract: Located in the central part of the Sunan Modernization Construction Demonstration Area—one of the areas with the densest population and urban areas and the most developed economy in China, the Liyang-Nandu fault is an important concealed fault with a nearly E-W strike in the Yangtze River Delta in eastern China. This fault serves as an important regional geomorphic and neotectonic boundary. However, there is a lack of systematic research on this fault, and its latest activity era is yet to be ascertained. Using seismic exploration P-waves and S-waves and cross-well stratigraphic correlation, this study investigated the spatial distribution and Quaternary activity of the Liyang-Nandu fault and explored its multiphase tectonic activity and seismicity. The results are as follows: (1) The western and central-eastern segments of the Liyang-Nandu fault have dip directions of north and south, respectively, suggesting the nature of a normal fault. Their dip angles range from 64° to 84°. (2) This fault evidently offsets the intervals above at burial depths of approximately 70 m, with vertical activity difference affecting the parts at depths of about 36-45 m underground. The comprehensive analysis revealed that this fault should be a Quaternary Early-Middle Pleistocene fault, with the latest activity occurring after approximately 310±54 ka, predating the Late Pleistocene. (3) This fault has undergone multi-stage activity. From the Indosinian to the Early Yanshanian, it was present as a reverse fault. Since the Late Yanshanian, it has been a normal fault, controlling the development of Cenozoic basins and volcanic rocks. These results will provide a scientific basis for the prevention and reduction of earthquake-induced hazards, major engineering construction, urban planning, and land use in southern Jiangsu and even the broader Yangtze River Delta.