We propose a new concept that has the potential to mitigate seawater intrusion and increase the fresh groundwater storage of oceanic islands by creating a less permeable slice along the shoreline. We present a proof‐of‐concept study to examine its effectiveness through analytical and experimental studies. Analytical expressions for calculating the freshwater‐seawater interface location, water table elevation, fresh groundwater volume, and groundwater travel time are presented for both barrier and circular islands, which are found dependent on three different scenarios of interface locations. The analytical solution of the interface location in a barrier island is verified through sand‐tank experiments. Sensitivity analyses based on a simplified conceptual model of St. George Island in Florida, USA, indicate that the fresh groundwater volume monotonically increases with the decrease in the hydraulic conductivity of the coastal less permeable hydrogeologic unit. On the other hand, the increase of the coastal less permeable unit extent leads to an increased fresh groundwater volume. However, when the interface tip is on the aquifer bed of the coastal less permeable unit, a further increase of the less permeable unit extent only slightly increases the fresh groundwater volume, since the interface does not change any more and only the water table is elevated. We demonstrate here that the concept proposed has the potential in increasing the fresh groundwater storage of oceanic islands. Analytical expressions presented can improve our understanding of seawater intrusion in a dual‐unit oceanic island.