Associating sexes in many species of the Neotropical hesperiid genus Porphyrogenes has been difficult, erroneous, or impossible, owing to their sexual dimorphism. Despite the extreme sexual dimorphism of Porphyrogenes peterwegei Burns, new species, full description of this rainforest skipper from Area de Conservación Guanacaste (ACG), northwestern Costa Rica, is no problem. The available sample amounts to nearly 100 reared adults, almost evenly divided between the sexes. Of these, 67 have been DNA barcoded and found to vary minimally in their COI haplotypes, with one haplotype predominating. Caterpillars and pupae consistently go through a distinctive set of developmental changes in their color patterns. Foodplant choice is conservative: of 227 immatures found in the wild, 94% were eating woody vines of the genus Machaerium, especially M. seemannii (Fabaceae), whereas 6% were feeding on the quite unrelated species Dichapetalum morenoi (Dichapetalaceae)—a peculiar pattern of larval foodplant selection seen in another skipper and two butterfly species in ACG. Caterpillars of P. peterwegei are occasionally attacked by a host-specific parasitoid (an undescribed tachinid of the genus Siphosturmia), which has not otherwise been encountered in 20,642 tachinid attacks on the caterpillars of >3,000 species of Lepidoptera in ACG. Morphologically, P. peterwegei differs significantly from its many congeners in both facies and genitalia.