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The first unique genus of ringtail possums from the Riversleigh World Heritage Area in Australia is diagnosed and described. Gawinga aranaea is the type and only species of the new genus and is known from nine isolated lower molars. It has been recovered from three Riversleigh deposits: two are of early to mid-Miocene age, while the age of the other has yet to be determined. The new possum is larger than Oligo-Miocene species of Paljara, Pildra, and Marlu, but smaller than most extant taxa. It is characterised by a distinctive lower molar cusp morphology of parallel ridges extending primarily from the cristid obliqua, filling the occlusal basins. Additional autapomorphies include: extended, shelflike protostylid cristids and a bisected posthypocristid on m1, and posterior molars that have a metaconid ridge posterobuccal to the metaconid and an anteriorly positioned protoconid relative to the metaconid. It also possesses an enlarged protostylid on m1, a feature otherwise known only in extant genera. The precise phylogenetic position of Gawinga within the pseudocheirid radiation has yet to be determined, but it is tentatively identified here to be a highly derived pseudocheirid, apomorphic with respect to Paljara and Pildra species, and with no known descendants. Copyright E American Museum of Natural History 2007 ISSN 0003-0082 1 School of Biological, Earth and Environmental Science, University of New South Wales, New South Wales 2052, Sydney, Australia ([email protected]). 2 School of Biological, Earth and Environmental Science, University of New South Wales, New South Wales 2052, Sydney, Australia; Research Associate, Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History ([email protected]). 3 School of Biological, Earth and Environmental Science, University of New South Wales, New South Wales 2052, Sydney, Australia; Research Associate, Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History ([email protected]). 4 School of Biological, Earth and Environmental Science, University of New South Wales, New South Wales 2052, Sydney, Australia ([email protected]).