Whollydooleya tomnpatrichorum gen. et sp. nov. is a new, highly specialised hypercarnivorous dasyuromorphian from a new mid-Cenozoic limestone deposit southwest of the Riversleigh World Heritage Area in northwestern Queensland. Dental dimensions suggest it may have weighed at least twice as much as the living Tasmanian devil (Sarcophilus harrisii). Although known only from a lower molar, it exhibits a plethora of carnivorous adaptations including a hypertrophied protoconid, tiny metaconid and a battery of vertical carnassial blades between most of the major cusps, most of which incorporate carnassial notches to immobilise materials being sheared. It is unique among dasyuromorphians in having a massive entoconid that closes the entire lingual side of the talonid. Comparison with previously known thylacinid and dasyurid hypercarnivores suggests its relationships are closer to dasyurids than thylacinids in the main because of the very large entoconid, a cusp that is relatively small to absent in all known thylacinids but commonly small to large in dasyurids. However, the extent of enlargement of the entoconid suggests that it is not closely related to previously known Cenozoic hypercarnivorous dasyurids in the genera Dasyurus, Glaucodon, Sarcophilus or any of the other previously described Cenozoic dasyurids. Although the early late Miocene Ganbulanyi djadjinguli is only known from an upper molar, the reduced area of its protocone suggests a correspondingly reduced rather than hypertrophied entoconid in its as-yet-unknown lower molars. Reconsideration of the structure of the talonid in species of Sarcophilus even suggests that within that Quaternary lineage, the entoconid may have been entirely lost, with the posteriorly displaced metaconid taking its functional place as an occlusal counterpart for the blades of the protocone. The large size of the new species signals the earliest indication within the dasyurid radiation of a late Cenozoic trend towards gigantism that became evident in many lineages of Australian marsupials. While the age is uncertain, on the basis of associated taxa such as species of Ekaltadeta, it is probably either mid or late Miocene in age. Geological features of the deposit suggest it was formed within a pool in a cave environment that periodically underwent desiccation. Some grains suggest an aeolian as well as an alluvial and pluvial origin for the deposit. This may relate to current understanding about environmental change that took place in the region following the mid Miocene climate oscillation.