A common mactrid from the southwestern Atlantic Ocean has been largely misunderstood in the literature. Since the early 20^th century, this species has been referred to as Mactra patagonica d'Orbigny, 1846. However, the latter is recorded in Quaternary deposits of Northern Patagonia. Mactra guidoi n. sp. is described herein, including its shell ultrastructure, hinge features, and soft parts, and compared with the holotype of M. patagonica. To compare and to correctly identify both species, a morphometric analysis was carried out using conventional and geometric morphometric methods. Mactra guidoi n. sp. has an elongate shell that is more fragile than that of M. patagonica. In addition, its ventral posterior edge is usually sinuous. The ultrastructure of the shell reveals two layers with cross lamellar structure. The anatomical features agree with those of the congeneric M. isabelleana, although they differ in the morphology of the labial palps and foot. Morphometric analysis revealed significant differences between M. patagonica and the new species. The relative warp analysis allows us to understand the variation recognized by using CVA. Only M. patagonica revealed significant allometry between Relative Warp and Centroid size. Finally, the comparison of the types clearly points towards the existence of two different species.