Morphological character data are inadequate to resolve the evolutionary relationships of the didelphid genus Chacodelphys, which previous phylogenetic analyses have alternatively suggested might be the sister taxon of Lestodelphys and Thylamys (tribe Thylamyini) or of Monodelphis (tribe Marmosini) in the subfamily Didelphinae. Because fresh material of Chacodelphys is unavailable, we extracted DNA from microscopic fragments of soft tissue adhering to the 95-year-old holotype skull of C. formosa. Phylogenetic analyses of the resulting sequence data convincingly resolve Chacodelphys as the sister taxon of Cryptonanus, a genus with which it had not previously been thought to be closely related. This novel clade (Chacodelphys Cryptonanus) belongs to an unnamed thylamyine lineage with Gracilinanus and Lestodelphys Thylamys, but relationships among these taxa remain to be convincingly resolved.