Nine specimens of retroplumid crabs collected from the late early or early middle Eocene Tallahatta Formation in southern Alabama form the basis for description of a new species, Costacopluma grayi. The discovery confirms the extension of the range of the genus into the Eocene and represents the first occurrence ofCostacopluma in the United States. As a result of the geologic range extension, the genus is now known to be contemporary with two other retroplumid genera, Retrocypoda, and Retropluma.