We report an isolated right lacrimal of a tyrannosaurid theropod dinosaur, probably from the Upper Cretaceous (upper Campanian) Judith River Formation of Fergus County, Montana. The lacrimal was originally associated with the holotype of the giant crocodylian Deinosuchus rugosus, but was later identified as that of a tyrannosaurid. It is of comparable size to the corresponding element in the gigantic Maastrichtian tyrannosaurid Tyrannosaurus. Moreover, comparison of the lacrimal to those of other tyrannosaurids tentatively supports its referral to this genus. Consequently, provided that its stratigraphic provenance has been correctly identified, the specimen represents the oldest-known North American record of a Tyrannosaurus-sized tyrannosaurid, and possibly the most ancient occurrence of this genus yet documented.