Genypterus valdesensis, sp. nov. from the Puerto Madryn Formation in Península Valdés, Argentina is the first fossil ophidiid reported from South America and represents the first well preserved record of the genus worldwide. The specimen consists of a nearly complete articulated skull and some disarticulated postcranial elements. A thorough description is given of the osteological features of the species, which can be used to recognize it as a distinct taxon. The fossil species is characterized by the presence of a foramen on infraorbital 3 and by the square-shape of infraorbital 4. An array of morphological features taken in combination contribute to further differentiate G. valdesensis from the Recent species. This specimen also provides unequivocal evidence of the presence of the Ophidiiformes in the southwestern Atlantic Ocean 14 MA.