Exquisitely preserved plant remains were identified from calcareous concretions recovered from the Beta Member (lower-mid Campanian) of the Santa Marta Formation, James Ross Island, Antarctic Peninsula. Plant remains were originally transported by estuarine currents from the east continental margin of the Peninsula and deposited in a shallow-marine environment. Several permineralized fruits were studied via serial acetate peels and from details exposed along the surface of concretions. Two small (4-8 mm) morphotypes were identified: dimerous (bilocular) and trimerous (trilocular) winged fruits, including a range of developmental stages. The pericarp of each morphotype is differentiated into three zones: exocarp, mesocarp, and endocarp. The fruits have two (dimerous) to three (trimerous) wings that are longitudinally elongate and formed by the outer layer. Two ovules with axile placentation are present in each locule. The locules lack trichomes. No distinct vascular bundles are seen in the mesocarp or in the central axis. There are, however, several vascular bundles running from the base of the fruit through the outer layer and toward the apex. Permineralized mature trimerous fruits are four times the size of the immature ones, with more developed sclerenchymatous mesocarp. The fruit symmetry, wings, general fruit shape, axile placentation, two ovules per locule, sclerenchymatous mesocarp, and glabrous inner locule wall are indicative of Nothofagaceae. The dimerous and trimerous fruits have the same tissue organization, indicating that both of these fruit types likely belong to the same taxon and were borne within a three flowered cupule (two trimerous flowers and one middle dimerous flower), as seen in most extant Nothofagus species. The fruits reported here are the first Cretaceous record for reproductive structures of the family, and are contemporaneous with the oldest pollen records (also from Santa Marta Formation) of Nothofagaceae that are assigned to ancestral pollen groups. In overlying Late Campanian-Maastrichtian deposits in Antarctica, the three extant Nothofagus subgenera are well represented by pollen, leaves, and wood. The structure of the fossil fruits studied here share all of the diagnostic characters of extant Nothofagus. Our findings support that three flowered cupule (with a middle dimerous flower) evolved early in the evolutionary history of the family.