Deltatheridium (meaning triangle beast or delta beast) is an extinct species of metatherian. It lived in what is now Mongolia during the Upper Cretaceous, circa 80 million years ago. It was a basal metatherian, which places it near start of the lineage that led to the marsupials, such as kangaroos, wallabies, koalas, and opossums. It had a length of 15 cm (5.9 in). Its teeth indicate it was carnivorous.