Cascadia’s 1700 earthquake generated a tsunami and widespread subsidence along Pacific Northwest coastline. The tsunami deposited sandy sediments in many Oregon coastal estuaries, such as Alsea Bay. Although the recurrence of subduction-generated megathrust earthquakes and tsunami events in this region are recorded in the stratigraphy, knowledge of the full inland extent of inundation is limited. A series of eighty-one 25-mm gouge cores taken throughout Alsea Bay show a widespread distribution of buried gray sand at depths of 0.29 to 0.73 meters beneath the overlying tidal-influenced mixed alluvium. These fine-grained tsunami sands, ranging in thickness from 0.6 to 13.4-cm, demonstrate landward thinning from the Pacific Ocean at distances of seven kilometers inland along the Alsea River. The data gained in this study further fills the spatial gap in knowledge concerning tsunami sediment lithology and inundation extent along the central Oregon coast between the Salmon and the Coquille Rivers. Identification of tsunamigenic sediments and characteristics of their distribution within estuaries and bays constrain existing and future modeling of coastline inundation following rupture of the Cascadia Subduction Zone.