A new, toothed ornithorhynchid monotreme from Two Trees Site in the Riversleigh World Heritage Area, northwestern Queensland, Australia, is described. This species is the largest known ornithorhynchid, fossil or extant, the fourth extinct platypus described, and the second species discovered at Riversleigh. It exhibits a unique molar morphology that significantly broadens understanding about disparity within this group of monotremes and challenges a previous presumption that fossil species of Obdurodon form an anagenetic lineage leading directly to the living dentally degenerate Platypus Ornithorhynchus anatinus. Apical cusp wear on its teeth suggests that it was crushing rather than cutting hard prey items. Its relatively derived features also adds to mounting evidence that the Two Trees deposit that contains several unique taxa may be younger than the surrounding middle Miocene fossil deposits, possibly late Miocene or even Pliocene in age, intervals of time previously unrepresented by ornithorhynchids.