Estuaries occupy a critical yet underappreciated position at the interface of riverine, coastal, and groundwater systems. While the impacts of climate change on marine coastal aquifers and estuarine surface waters are well documented, comparatively little attention has been given to the vulnerability of soils and groundwater adjacent to estuaries. These areas are often low-lying, densely populated, and agriculturally productive, making them particularly sensitive to compounding climate stressors. Here, we synthesize global datasets, conceptual models, and existing field and numerical studies to assess the extent and mechanisms of soil and groundwater salinization near estuaries. A first-order geospatial analysis reveals that approximately 150,000 km² of agricultural land and numerous major population centers are located within low-elevation coastal zones close to estuaries, highlighting disproportionate exposure. Conceptual and modeling evidence indicates that rising estuary water levels and salinity, driven by sea-level rise, storm surge, drought, and altered river discharge, can promote both lateral and vertical subsurface saltwater intrusion. Unlike marine coastal settings, estuarine environments exhibit strong spatial and temporal variability in surface water salinity and hydraulic gradients, resulting in complex groundwater–surface water interactions that challenge conventional coastal vulnerability assessments. These dynamics can extend saline influence inland through hydraulic short circuits, affecting soils, freshwater aquifers, agricultural productivity, and water supplies in areas not traditionally considered at risk. This synthesis highlights key knowledge gaps related to monitoring, modeling, and management of estuary-adjacent groundwater systems and underscores the need to explicitly include these environments in coastal adaptation and water-resource planning under a changing climate.