A new species of the ptilodontid multituberculate genus Prochetodon (Mammalia, Allotheria) from the Paleocene Paskapoo Formation of Alberta, Canada, is described. Prochetodon speirsae sp. nov. is the oldest discovered species of the genus, extending the temporal range of Prochetodon earlier into the Paleocene. It exhibits a mosaic of primitive and derived dental features suggestive of a position phylogenetically intermediate between Ptilodus and Prochetodon. Newly discovered specimens from the late Paleocene Gao Mine locality are referred tentatively to Prochetodon foxi. The occurrence of a primitive form of Prochetodon in sediments of early Tiffanian age suggests the evolutionary history of the genus is more complex than previously appreciated. Prochetodon joins a number of well-documented, progressive mammalian taxa making their first appearance in the Western Interior of North America at higher latitudes, well before their first known occurrence in the United States, suggesting a possible origin and immigration from currently unsampled habitats.