Billfishes are characterized by the elongation of their upper jaw into a bill or rostrum, this structure has a relevant role in feeding. Differences in rostrum morphologies have been linked to a range of different feeding behaviors and biomechanics. While the role the rostrum plays in prey processing has been thoroughly investigated, our understanding of the morphology and biomechanics of other feeding structures is less understood. The morphology of the lower jaw in billfishes is diverse among species, and one of the most interesting characteristics is the presence of the predentary, a distal bone unique to Istiophoridae (marlins and sailfishes) but absent in all extant fishes including their sister group, Xiphidae (swordfishes). We investigated the role of the predentary in billfish feeding using Finite Element Analysis, hypothesizing that the predentary reinforces the symphysis of the more robust istiophorid jaws to prevent torsion and “wish boning”, thus allowing Istiophorids to withstand the larger bite forces they are known to exert relative to the Xiphiids. Finite Element models of the lower jaws of multiple billfish species were constructed from CT scans, and Von Mises stresses were compared. Different predentary morphologies, predentary presence and absence and bite force based loading regimes were compared. Results show lower stresses along the lower jaws of the blue marlin during unilateral loading, indicating that the predentary could help resist torsion. Indeed, virtual removal of the predentary bone modified the patterns of lower jaw stress. The functional implications of the predentary, in addition to other morphological differences between the lower jaws among billfishes, give further insight in to the differences in the group’s feeding biomechanics and behaviors.