Coastal wetlands form at the dynamic interface between terrestrial and marine environments. To date, most research has focused on ocean inundation as a driver of ecosystem productivity, health, and biogeochemical cycling. While the ocean’s influence is undeniably important, terrestrial aquifers also interact with coastal wetlands along their upland and subsurface boundaries. However, the role of terrestrial groundwater in modulating coastal wetland ecosystem dynamics has scarcely been investigated, potentially limiting the efficacy of ecosystem projections in a changing future. Here, we investigate drivers of coastal wetland redox potential, a key indicator of sediment biogeochemistry, reflecting oxic/anoxic transitions that shape wetland resilience. Using wavelet and mutual information analyses at seven coastal wetland sites across the United States, we evaluate the influence of wetland water level, terrestrial groundwater level, and meteorological factors on coastal wetland redox potential. Results show that, across all sites, terrestrial groundwater level was the dominant control on coastal wetland redox potential. The strong link between terrestrial aquifers and coastal wetland redox potential observed here contrasts with the traditional paradigm of ocean inundation as the primary ecosystem modulator and suggests that coastal aquifers are a key driver of biogeochemical processes. This upland-marsh connectivity also indicates that changes in coastal aquifers, such as drought or pumping, may affect the biogeochemical conditions and future resilience of coastal wetlands.