Secondary sexual dimorphism is relatively common in the ghost knifefishes (Gymnotiformes: Apteronotidae), with males of several species growing greatly elongated snouts (e.g., Compsaraia samueli, Parapteronotus hasemani), while others develop enlarged, protruding teeth on either the lower jaw (Sternarchorhynchus spp.) or both jaws (Sternarchogiton nattereri). Of the four known species of Sternarchogiton, sexual dimorphism has so far only been reported in S. nattereri. Here we report that in an additional species, S. labiatus, mature males possess similar enlarged, external teeth on the dentary and premaxilla. We document this condition in three specimens collected during the high-water spawning season from the río Nanay near Iquitos, Peru. We analyze this morphology using high-resolution X-ray microcomputed tomography. Additionally, we use genetic sequence data to demonstrate that specimens bearing external teeth are genetically indistinguishable from those with the phenotype of S. labiatus. Finally, we review and summarize the current knowledge of sexual dimorphism within the Apteronotidae.