Lanternfishes (Myctophiformes, ~260 species) are among the most abundant and species-rich groups of fishes endemic to the deep sea, and they play a major role in the oceanic ecosystem by transferring energy from shallower to deeper oceanic levels. Little is currently known regarding how lanternfishes have achieved such high species richness in the deep sea, and the majority of previous studies that have investigated diversification in this group have focused on bioluminescence. In this study, we investigate the variation in mouth size of lanternfishes in an effort to better understand potential mechanisms of speciation in this group, as previous studies have indicated that there is considerable variation in the size and biodiversity of prey items of lanternfishes. Geometric morphometrics were performed on 955 lanternfish specimens, and an ancestral character-state reconstruction was used to examine variation and evolution of mouth size in this group. We identify that mouth size in lanternfishes is highly variable, with general trends towards larger mouths in the subfamily Lampanyctinae (Myctophidae) and shorter mouths in the subfamily Myctophinae (Myctophidae). Within each subfamily there are discrete patterns of jaw-size differentiation among genera. Of particular note, the genus Diaphus, the most species-rich genus of lanternfishes (~30% of lanternfish diversity), was found to occupy a large range of morphospace, with broad plasticity in mouth size among the examined species. Ancestral character-state reconstructions indicate that a neoscopelid-like jaw was the likely ancestral state for Myctophiformes; whereas, a longer jaw, similar to that of the majority of species in the subfamily Lampanyctinae, was most likely the ancestral state for the family Myctophidae.