Continental interiors were flooded by epeiric seas during many intervals of the geologic past. Few modern analogs exist for these environments, however, and basic variables such as redox, salinity, and restriction are difficult to reconstruct in deep time. Despite these challenges, constraining epeiric watermass properties is critical because much of our preserved and accessible sedimentary record was deposited in such settings. Here, we present a four‐dimensional reconstruction of watermass evolution in the Late Devonian Appalachian Seaway of North America. We use combined proxies for sediment supply, paleosalinity, paleoredox, and basin hydrography in six cores through the Upper Devonian Cleveland Shale deposited across a paleo‐depth transect. Cyclic, coupled changes in sedimentation, redox, and salinity are recorded in environments near the Catskill Delta. Additionally, a pronounced salinity gradient was present from low‐brackish conditions near the delta to fully marine conditions in the basin interior, with a lower‐salinity mixing zone recorded across the Cumberland Sill. We also identified two broad sequences—the lower and upper Cleveland Shale—each of which shows distinct watermass signatures. The lower Cleveland Shale records a redox gradient with euxinia only present along the Cumberland Sill, whereas the upper Cleveland Shale records intensification of euxinia (potentially in the photic zone) at all six sites, which may be coincident with the Hangenberg extinction event. Ultimately, this study identifies pronounced epeiric watermass gradients over short timescales (millennia) and distances (hundreds of km or less), highlighting the need for interpreting the geochemistry of epicontinental deposits in the context of basin hydrography and paleosalinity.