Abstract Offshore Freshened Groundwater (OFG) reservoirs are regarded as a potentially significant resource stored in continental shelves. While the role of subsurface geological conditions in controlling OFG has been widely recognized, the influence of the topography of the hosting continental shelf on OFG evolution remains poorly understood. Here, using generalized numerical modeling representative of a broad range of systems worldwide, we evaluate how the shelf slope affects the evolution of OFG in subsea aquifers, considering sea‐level rise over paleohydrogeological timescales. Our findings show that shallow dipping shelves are expected to have greater extents of OFG by up to 2.8 times and enhance the longevity of OFG preservation. Crucially, natural‐scale alongshore slope variability substantially influences the cross‐shore salt distribution. Considering alongshore flow shows that cross‐sectional analyses can overestimate OFG volumes by up to 3 times. These findings highlight the significance of shelf slope and lateral (alongshore) dynamics in estimating and predicting OFG resources.