Redescribed and reinterpreted here is a partial skeleton of the oldest Antarctic penguin, Crossvallia unienwillia from the late Paleocene of Marambio (Seymour) Island. A thorough morphological analysis of limb bones (humerus, femur and tibiotarsus) allowed us to report both an amended reconstruction of type specimens and a revised taxonomic diagnosis of this large-sized early penguin. Moreover, an additional skeletal element, a single thoracic vertebra, is described for the first time. C. unienwillia is characterized by a mixture of features typical of both geologically older and younger penguins, supplemented by characters that are obviously unique. The above-mentioned findings shed new light on the very beginning of penguin evolution.