Here, we review the species Cratomorphus cossyphinus (Perty, 1830) after study of the holotype, 24 adults and nine immature stages deposited in five collections. The species is found throughout South America, inhabiting wetlands. Larva, pupa and female are described for the first time, and the male is redescribed and illustrated, with an emphasis on sexual dimorphism. The larva is considered semiaquatic, condition also known in related Cratomorphini taxa, found on vegetation either marginal or over sand islets on marshes. Sexually dimorphism was more pronounced in the size and shape of the following structures: frons, antenna, eyes, hypomeron, hind wing, lantern on the sternum VI, sternum VIII and pygidium. Interestingly enough, most of the aforementioned features are involved with mate finding. We also report remarkable multiple spermathecae in the female, unprecedented for the genus and for the family as well. Finally, we provide illustrations and diagnoses for all stages, and an updated distribution map for the species for South America, reporting C. cossyphinus from Venezuela and Suriname to Southern Brazil.