At the end of the Early Cretaceous the once abundant sphenodontians vanished from the Laurasian record and were thought to have become virtually extinct, with the sole exception of Sphenodon, the living tuatara. Recent findings of large and abundant eilenodontine sphenodontids in the Early Late Cretaceous (Cenomanian-Turonian) and fragmentary material from other lineages from Late Campanian outcrops of Patagonia, Argentina, have demonstrated that sphenodontids constituted an important component of the Late Cretaceous terrestrial ecosystems in South America and possibly Gondwana. Although eilenodontine and possibly sapheosaurine sphenodontids are present in the Late Cretaceous of Gondwana, they were only part of an unknown southern radiation. We report here on a new sphenodontid, Lamarquesaurus cabazai, n. gen. et sp., which is represented by an incomplete right maxilla that represents a previously unknown non-eilenodontine lineage and illustrates the diversity and role of sphenodontians in the tetrapod communities of the Late Mesozoic of South America.