A locality specific, species level analysis of Pediomyidae from the Late Cretaceous of South Dakota, indicates the presence of new taxa. The research focuses on a suite of fossilized upper and lower molars of the extinct marsupial family Pediomyidae, recovered from the Red Owl Quarry in the Fox Hills Formation of western South Dakota. Pediomyids were an early lineage of marsupials that originated in North America during the Maastrichtian age (72.1-66 mya) in the Late Cretaceous. This study of new pediomyid fossils will be the first published identification and description of marsupial fossils from the Fox Hills formation. To date, the Fox Hills has produced a variety of marine remains and dinosaurian specimens but mammals have yet to be identified and described from this formation. The upper molars, in the third position, provide the greatest phylogenetic resolution due to distinct generic characteristics. Lower molars, first and second molars tend to be less diagnostic at the generic and species level. Specimens are identified based on their shared characteristics with currently accepted pediomyid taxa and new species delineated by their derived traits and their relations to current phylogenies. Software programs TNT and Mesquite were used for all analyses. The results indicate two previously undocumented genera present at the Red Owl Quarry. Additionally, the assemblage of Late Cretaceous marsupials contains taxa that are morphologically distinct from those of the older Judithian North American Land Mammal Age (NALMA) (83.5-70.6 mya) and the younger Lancian age (70-66 mya). The division between the Judithian and Lancian is still debated and for many years paleontologists have been trying to identify distinct land mammals that will help define the “Edmontonian” as a separate, intermediate geologic age from that of the Judithian and Lancian. The presence of temporally distinct taxa adds to the mammal diversity needed to establish the “Edmontonian” as a recognized age. There have been too few distinct mammalian faunas in which to formally define the age therefore the Fox Hills formation containing mammalian fossils is significant for Late Cretaceous studies as being the first published description of land mammals from an otherwise marine formation.