Newly discovered mandibles and lower dentition of the middle Eocene rodent Thisbemys brevicrista from the Green River Basin, Wyoming, are the basis for a species re-diagnosis. Previously, only the upper dentition and a partial maxilla of T. brevicrista were known from Br3. New specimens from Br2 now include the lower molars, additional upper molars, and maxillary fragments including a partial zygomatic arch that preserves the relationship of the arch to the first upper molar. In addition, the presence of T. brevicrista at Br2 documents the co-existence of T. brevicrista with T. perditus, T. nini, T. plicatus, and T. corrugatus. Formerly, the latter two species were differentiated primarily using stratigraphy. Now, morphology and size can also be used. Thisbemys brevicrista is intermediate in size between T. plicatus and T. corrugatus, and can readily be distinguished from these species based on unique features of the upper and lower molars, which include an additional loph on the two anterior upper molars and a complete metalophid on m1–3. The original type specimen of T. brevicrista appears to be lost, thus a neotype is designated as part of the re-diagnosis. Discovery and description of the lower dentition and mandibles of T. brevicrista clarifies the alpha taxonomy of this species, and increases its utility for studies of phylogenetic relationships and for documenting Eocene mammalian diversity patterns.