Collecting by Alcide D'Orbigny in the western South Atlantic provided material for Platessa orbignyana Valenciennes, 1839, published as an illustration of the holotype without accompanying descriptive information. A brief and inadequate description was provided in 1847. Thereafter, the holotype was thought to be lost, precluding comparative study and resulting in nomenclatural confusion for other nominal paralichthyid species. A specimen of Paralichthys collected in the Río de la Plata, Buenos Aires, Argentina by D'Orbigny at the Muséum national d'histoire naturelle (MNHN 1999-0295) is here identified as the holotype of Platessa orbignyana Valenciennes, 1839. A redescription of the holotype and comparison with other South Atlantic species resolves several nomenclatural issues.