Proechimys roberti has been considered an isolate representative of the guyannensis group of species of the genus Proechimys in the Cerrado of central Brazil, or regarded as a member of the longicaudatus group of species. We present new karyologic, morphometric, morphologic, and molecular data from populations spanning the gap between the previously known isolated distribution of P. roberti and the core distribution of the guyannensis group. All specimens had the same basic karyotype, 2n = 30, with fundamental number (FN) = 54–55. FN variation was due to a pericentric inversion affecting a small autosomal pair. Specimens from the eastern Amazon Basin and the Cerrado did not have any differentiation in external, cranial, and dental characters. In contrast, samples showed a significant multivariate morphometric variation, with a high heterogeneity among all populations, independent of Cerrado or Amazon location. Our results, coupled with phylogenetic analysis of cytochrome-b sequences, indicate that P. roberti is a valid species of the guyannensis group and that the species of this group occurring in eastern Amazon, P. oris, is probably a junior synonym of P. roberti.