Mangroves cover ∼240,000 km² of sheltered subtropical and tropical coasts and provide numerous ecological functions and related goods and services. Being located at the immediate terrestrial-marine interface, the structure and function f mangroves is strongly controlled by soil porewater salinities. The objective of this study was to characterize the distribution and controls on soil porewater salinity in mangroves in protected yet tidally influenced environments. The study site was located on a barrier island bounding the Indian River Lagoon, a 250-km long estuary on the east-central coast of Florida. Water levels in the mangrove were almost always lower than water levels in the lagoon. Though water levels in the lagoon were tidally influenced, spectral analysis showed that water levels in the mangrove were not tidally influenced and were instead more strongly controlled by diurnal patterns related to evapotranspiration. Soil porewater salinities varied spatially, with values of ∼10 psu in uplands, ∼30 psu in regularly-flushed mangroves, and ∼75 psu in irregularly-flushed mangroves. Cation and anion concentrations and stable isotope compositions indicated that soil porewater salinities were largely controlled by enrichment due to evapotranspiration. A shore-perpendicular electrical resistivity survey showed that a freshwater lens was restricted to uplands and that hypersaline soil porewaters extended deeply below the mangrove. These results indicate that evapotranspiration lowers water levels in the mangrove, which causes lagoon water to flow into the mangrove where it evapoconcentrates and descends, forming a thick layer of high-salinity soil porewater below the mangrove.