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Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and SO₄-reducing conditions in previously oxic-acidic sediments. This creates potential for mobilization of As during the redox transition. We explore the consequences for As by investigating the hydrology, porewater geochemistry, solid-phase speciation, and mineralogical partitioning of As across two tidal fringe toposequences. Seawater inundation induced a tidally controlled redox gradient. Maximum porewater As (∼400 μg/L) occurred in the shallow (<1 m), intertidal, redox transition zone between Fe-oxidizing and SO₄-reducing conditions. Primary mechanisms of As mobilization include the reduction of solid-phase As(V) to As(III), reductive dissolution of As(V)-bearing secondary Fe(III) minerals and competitive anion desorption. Porewater As concentrations decreased in the zone of contemporary pyrite reformation. Oscillating hydraulic …