The New Zealand tuatara (Sphenodon) is the sole surviving rhynchocephalian of a once thriving group across Pangea during early Mesozoic times. Outside New Zealand, close relatives of the tuatara (sphenodontines) are known from a few Jurassic records in North America and Europe and from end-Cretaceous incomplete remains in Patagonia. Still, the evolutionary relationships of most of them remain elusive. Here we describe a new sphenodontine, Tika giacchinoi gen. et sp. nov., based on well-preserved cranial and postcranial remains from upper levels of the Candeleros Formation (Cenomanian) at the Konservat-Lagerstätte of ‘La Buitrera Paleontological Area’ in northern Patagonia, Argentina. Our phylogenetic analysis recovered Tika as a close relative of the tuatara, together with Laurasian and Patagonian taxa. The new finding represents the oldest certain sphenodontine from the gondwanan continents and reinforces the hypothesis that particular terrestrial ectothermic tetrapods attained a circumantarctic Cretaceous-Tertiary distribution. Tika is inferred to have fed upon a variety of prey items including small vertebrates, similar to the extant tuatara, but ecologically different from the large herbivorous sphenodontians already known from La Buitrera. Therefore the new taxon expands the known diversity of sphenodontians during the Late Cretaceous in Patagonia and indicates that Rhynchocephalia, although declining or extinct in Laurasia, were still taxonomic and ecologically diverse in southwestern Gondwana.