Five nominal species of small didelphid marsupials previously referred to Gracilinanus differ conspicuously from the type species (G. microtarsus) and from all of the other valid taxa that we recognize as members of that genus (G. aceramarcae, G. agilis, G. dryas, G. emiliae, G. marica). These anomalous forms can be distinguished morphologically from Gracilinanus (in the strict sense just defined) by lacking maxillary palatal vacuities, a secondary foramen ovale, and a rostral process of the premaxillae; in addition, P3 is taller than P2, and accessory cusps are often present on C1. A new genus, Cryptonanus, is described to contain these forms, all of which are provisionally recognized as valid species: C. agricolai, C. chacoensis, C. guahybae, C. ignitus, and C. unduaviensis. Separate and combined phylogenetic analyses of nonmolecular data and nuclear gene sequences suggest that Cryptonanus and Gracilinanus (sensu stricto) are reciprocally monophyletic and closely related, although they were not consistently recovered as sister taxa in any analysis. Available specimen records document that Cryptonanus is widely distributed in mostly unforested tropical, subtropical, and temperate biomes south of the Amazon River (from ca. 7°S in the Brazilian state of Ceará to ca. 34°S in the Argentinian province of Buenos Aires), but significant range extensions could be expected from pitfall trapping in extralimital savanna landscapes. Scant field data suggest that species of Cryptonanus may often be associated with wet or seasonally inundated grasslands, an unusual habitat for small didelphids.