Examination of genetic data (mitochondrial cytochrome c oxidase I) for western Atlantic clingfishes revealed two distinct lineages within a group of individuals originally identified as Acyrtus artius. Subsequent investigation of preserved voucher specimens was conducted to reconcile the genetic data and the existing classification, which is based on morphology. In addition to discovering that one of the genetic lineages is an undescribed species, which we describe as Acyrtus lanthanum, new species, we found that the nominal species Acyrtus artius has a putative venom gland associated with the subopercle that has been overlooked since the species was described nearly 60 years ago. The new species lacks the subopercular gland as does Acyrtus rubiginosus, but one is present in the related Arcos nudus. Venom glands have not been reported previously for the Gobiesocidae, and the venom gland described herein for Acyrtus and Arcos represents the first example in teleost fishes of a venom gland associated with the subopercle.