Members of the didelphid marsupial genus Monodelphis have previously been classified into species groups, but such informal subdivisions are inconsistent among authors and have contributed little but confusion to the taxonomic history of the genus. Recent molecular phylogenetic studies, however, have consistently recovered several strongly supported multispecies clades that merit formal recognition. Here we define and illustrate morphological characters that are useful for diagnosing these monophyletic groups, which we name and rank as subgenera to conserve binomial usage: Monodelphis Burnett, 1830 (for M. arlindoi, M. brevicaudata, M. domestica, M. glirina, M. palliolata, M. sanctaerosae, and M. touan), Microdelphys Burmeister, 1856 (for M. americana, M. gardneri, M. iheringi, and M. scalops), Monodelphiops Matschie, 1916 (for M. dimidiata and M. unistriata); Mygalodelphys, new subgenus (for M. adusta, M. handleyi, M. kunsi, M. osgoodi, M. peruviana, M. pinocchio, M. reigi, and M. ronaldi); and Pyrodelphys, new subgenus (for M. emiliae). We provide morphological diagnoses and describe pairwise comparisons to facilitate subgeneric identification, and we summarize subgeneric patterns of geographical distribution and sympatry to supplement recently published biogeographic analyses of the genus.