Hidden paleo-channels beneath the Changjiang Delta (CD) remain poorly constrained in their spatial extent and internal architecture. Paleo-channels that are infilled with high permeability sediments act as preferred pathways for groundwater flow and solute transport, providing a hydraulic connection for groundwater between land and sea facilitating saltwater intrusion (SWI) landward, or submarine groundwater discharge offshore. Although paleo-channels have been implicated in several instances of SWI, there has been little direct study of these channels in the CD over regional scales. Electromagnetic data that extend onshore hydrogeologic information offshore can play a key part in understanding the effects of paleo-channels on freshwater/seawater exchange. Here, we present first results from a large-scale (tens of kilometers) transient electromagnetic (TEM) investigation of the Shanghai coastal aquifer system on the southern flank of the CD, China. Previous small-scale (hundreds of meters) ground penetrating radar and transient electromagnetic surveys across the Nanhui wetland and tidal flat system provide initial constraints on shallow freshwater/saline groundwater transitions near the land-sea interface. Here, we extend these nearshore observations to the regional-scale with the goal of mapping geoelectrical structures both within and outside previously identified paleo-channel boundaries at depths of up to ~200 m. Preliminary 1-D inversion results reveal two prominent geoelectrical units. A laterally continuous low-resistivity layer at ~7.5–30 m depth, observed both inside and outside the paleo-channel boundary, correlates with borehole-defined confining beds. A deeper high-resistivity layer at ~50–70 m corresponds to the second confined aquifer. Localized low-resistivity patches occur within the paleo-channel region, and these anomalies may reflect saline groundwater storage, paleo-channel heterogeneity, or preferential zones for groundwater flow. Although a continuous land-sea pathway cannot yet be directly resolved, the observed resistivity patterns provide initial boundaries on where deeper groundwater discharge pathways may occur. This study provides new estimates on potential deeper groundwater pathways and offers a basis for future comparison with offshore freshened groundwater observations and groundwater sustainability assessments that can inform management decisions for the coastal megacity of Shanghai.