We report on the discovery of a remarkable defensive specialization in stonefishes that was identified during a phylogenetic study of scorpionfishes and their relatives. This newly described innovation, the lachrymal saber, involves modifications to the circumorbitals, maxilla, adductor mandibulae, and associated tendons. At its core, the lachrymal saber is an elongation of an anterior spine (or spines) on the ventral surface of the lachrymal that stonefishes are capable of rotating from the standard ventral position to a locked lateral position. The locking mechanism minimally includes a bony spur on the inner surface of the lachrymal and a ridged bony protuberance on the anterolateral end of the maxilla. A modified and highly subdivided adductor mandibulae appears to control the movement of the lachrymal saber by rotating the maxilla while it is engaged with the spur on the medial side of the lachrymal. This maxillary rotation results in a subsequent rotation of the lachrymal that we hypothesize reduces predation on stonefishes. This specialization was included in our phylogenetic analysis of scorpaenoid fishes. This study expands upon the previous higher-level taxonomic sampling reported in earlier evolutionary studies of scorpaenoid fishes and, unlike previous analyses, explicitly combines molecular and morphological data with an expanded taxonomic sampling to mitigate the conflict between these competing datasets. The resulting phylogeny based on a combination of 113 morphological and 5,280 molecular characters for 63 species is used to produce a revised taxonomy of flatheads, scorpionfishes, sea robins, and stonefishes. Our results do not support the monophyly of the traditional Scorpaeniformes, Scorpaenoidei, Scorpaenoidea, Platycephaloidea, Bembridae, Scorpaenidae, Sebastidae, Serranidae, Tetrarogidae, or Triglidae. Our monophyletic taxonomy recognizes nine monophyletic families: Bembridae, Congiopodidae, Hoplichthyidae, Neosebastidae, Platycephalidae, Plectrogeniidae, Scorpaenidae, Synanceiidae, and Triglidae. The taxonomic composition of the Congiopodidae, Hoplichthyidae, Neosebastidae, and Platycephalidae are unchanged. The Bembridae is expanded to include the recently described Parabembridae, while Bembradium is moved to the Plectrogeniidae. The Scorpaenidae is expanded to include the traditional Sebastidae and Setarchidae. The Triglidae is expanded to include the Peristediidae. Finally, a revised Synanceiidae, diagnosed by the lachrymal saber, is expanded to include the Apistidae, Aploactinidae, Eschmeyeridae, Gnathanacanthidae, Pataecidae, Perryenidae, and Tetrarogidae. Based on these results, we recommend treating all of these traditional scorpaenoid clades as families in an expanded Scorpaeniformes that includes a restricted Scorpaenoidei that includes all traditional scorpaenoid families except the Congiopodidae. The resulting phylogeny is then used to explore aspects of scorpaenoid evolution.