The new plant bug Eccritotarsus eichhorniae is described from specimens taken on water hyacinth, Eichhornia crassipes, in Maynas Province, Peru. This new species is extremely similar to Eccritotarsus catarinensis (Carvalho), a species described from Santa Catarina, Brazil. These two populations were considered conspecific until COI sequencing data and breeding studies indicated that the Peruvian material probably represented a new cryptic species. Subsequent morphological studies confirmed that two species were involved. In this paper diagnoses, descriptions, and illustrations of the adults, scent glands, and male genitalia are provided to distinguish these two closely related species, both of which are currently being used as biocontrol agents on water hyacinth in South Africa.