Seawater intrusion happens when saltwater moves into underground freshwater sources near the coast. Normally, this is caused by the natural difference in density between seawater and freshwater, and it can get worse when people pump too much groundwater or when sea levels rise. It can also happen when ocean water floods the land during storms. In this study, we identify a new way that seawater can move inland—not just downward from the surface, but also sideways from the coast. Using a 3D computer model, we found that when seawater floods land areas like canals or low-lying wetlands, it can act like a “magnet,” pulling more seawater underground and further inland. The distance that floodwater travels inland plays a big role in how far the saltwater spreads underground. This is important because with rising sea levels and more intense storms, coastal flooding is becoming more common. Our findings show that even small, local floods can have much bigger impacts on underground freshwater supplies than previously thought.