For over a century, the baenid turtle Boremys has been recognized as being restricted to the Campanian of North America. Herein we describe new material of Boremys sp. from the Hell Creek Formation (Maastrichtian) and Fort Union Formation (Puercan) of southwestern North Dakota and eastern Montana, increasing the stratigraphic range of this taxon by 11 million years. The material was recovered from the base of the Hell Creek Formation to 14 m above the pollencalibrated K/T boundary in the basal Fort Union Formation. Most of the specimens consist of isolated shell elements, which are easily misidentifled as belonging to a kinosternid or chelydrid turtle, but complete shells are present as well. The presence of Boremys sp. in the Hell Creek formation increases the baenid taxonomic diversity of this particular rock unit to nine and the overall turtle diversity to 20 taxa, and the presence of Boremys sp. in the Fort Union Formation increases the number of baenid lineages that survive the K/T extinction event to eight.