Eucosma oraria, new species, is described from the mid-Atlantic coast of North America. It is distinguished from its closest congeners, Eucosma dorsisignatana (Clemens) and Eucosma similiana (Clemens), by size and details of forewing maculation. Reviews are provided of the last two species, including a reevaluation of their relationships with Eucosma dorsisignatana diffusana Kearfott, Eucosma dorsisignatana confluana Kearfott, and Eucosma engelana Kearfott. The new species appears to be associated with marsh habitat.