Study of a nearly complete skeleton of a small amniote, preserved as part and counterpart in Upper Carboniferous limestone slabs of the Hamilton Quarry, Kansas, indicates that it is a new genus and species of varanopid synapsid. Archaeovenator hamiltonensis gen. and sp. nov. is the oldest known varanopid. Phylogenetic analysis indicates that it is the sister taxon to all other known varanopids. Discovery of this new varanopid is important because it improves our knowledge of the terrestrial fauna of the Hamilton quarry, expands the known diversity of Late Carboniferous basal synapsids, and extends the fossil record of Varanopidae deeper within the Stephanian. Varanopidae has the longest fossil record (Late Carboniferous – upper Middle Permian) of all known clades of basal synapsids.