In March 2012 and April—May 2013, we collected three dead specimens of Elliptio complanata (Eastern Elliptio) in the tidal, freshwater portion of the Patapsco River, providing the first evidence of this species from the river. Although these specimens do not definitively confirm the presence of an extant population, they indicate that a remnant population may well persist in the tidal, freshwater portion of the river. If it exists, this population could serve as a source for natural recolonization into the non-tidal reaches of the river where the species is believed to have once occurred but is now likely extirpated. Further mussel surveys are needed in the tidal, freshwater portion of the river to confirm the presence of an Eastern Elliptio population, and to determine its size, age structure, and distributional extent.