Comparative studies have shown that primates known to consume biomechanically tough and/ or hard foods have larger temporal fossae than primates that consume biomechanically softer foods. The former group also possesses relatively more robust zygomatic arches and exhibits increased cortical bone relative to total subperiosteal bone in the anterior zygomatic root to facilitate ef􀜪cient mastication. However, less is known about potential phylogenetic constraints on the internal and external morphology of the zygomatic arch. This study builds on previous work by evaluating if closely-related primates that differ in diet toughness and hardness diverge in functionally informative aspects of zygomatic arch morphology. Species from three genera were analyzed: 1) Gorilla gorilla and Gorilla beringei, 2) Pongo pygmaeus and Pongo abelii, and 3) Macaca spp. (seed eating macaques, non-seed eating macaques, and bamboo eating macaques). External and internal zygomatic arch morphology was quanti􀜪ed from linear measurements derived from 3D landmarks and cross-sectional data obtained from CT scans. Metrics did not statistically differ between species of Gorilla or Pongo, which might be explained by dietary variation among populations within each of these species. Seed eating macaques possess relatively more robust zygomatic arches with larger temporal fossae than other macaque groups. Relative cortical area distributions were highest in macaques that consume bamboo, and lowest in seed eating macaques, suggesting that loading regimes vary within biomechanically challenging diets. These results support the utility of external zygomatic arch morphology as an indicator of diet type that may be informative for interpreting the primate fossil record.