Assateague Island is a classic example of a wave-dominated, retrograding barrier island, with its recent geological history punctuated by episodes of overwash and breaching (comparable to the Outer Banks, North Carolina; northern New Jersey coast; Long Island, New York, etc.). This study aims to provide high-resolution data of a paleo-inlet which may lead to a more precise evolutionary model of other wave-dominated barrier islands. The study focuses on a historically stable segment fronting the Green Run Bay. The site lies north of the historical Green Run Inlet (active until 1880); however, there is no morphological or historical evidence of this inlet occupying the more northerly position, thus suggesting that the study site may represent a relict part of the barrier. High-resolution ground-penetrating radar (GPR) images, complemented with sediment cores and multi-dating techniques, were used to reconstruct the geological legacy of the Green Run Bay segment. The findings indicate that a shallow backbarrier channel, still visible within the Green Run Bay, can be traced to a large (> 380 m wide, 3.0–3.5 m thick) channel cut-and-fill structure beneath the barrier. The channel fill consists of tangential- to sigmoidal-oblique, southward-dipping reflections downlapping onto channel lag facies, which overlie subhorizontal bay-fill strata. Mollusks from the bay fill yield calibrated ages of 4419–2018 yr BP. The paleo-channel facies overlying the bay deposits exhibit a fining-upward sequence, with a mean grain size decreasing from 0.44 to 2.43 φ (0.74 to 0.19 mm). Optically stimulated luminescence dating places the middle part of the inlet fill at 730–590 yr BP. The paleo-channel fill does not extend to the south and therefore is a separate relict feature that predates the historical Green Run Inlet. The paleo-tidal prism of the relict channel was at least 17 × 106 m3, which is comparable to many wave-dominated historical and active inlets along the Atlantic Seaboard. Findings in this study provide the geological background for coastal hazard assessments and offer a mesoscale example of inlet-facies architecture in wave-dominated barrier sequences identified in the rock record.