The Plio-Pleistocene cave site of Sterkfontein has yielded the remains of Cercopithecoides williamsi, a colobine monkey reconstructed as a terrestrial folivore given its dental morphology, heavily worn teeth and robustly built postcranial skeleton. To address the validity of these dietary and habitat inferences, patterns of dental microwear obtained from low-magnification stereomicroscopy for C. williamsi specimens from Sterkfontein Member 4 (n = 13) are compared to a broad comparative sample of extant primates, including Cebus apella (n = 10), Colobus polykomos, (n = 6), Papio anubis (n = 13), and Papio ursinus (n = 39) as well as extinct Parapapio broomi (n = 12) from Sterkfontein Member 4 and Theropithecus oswaldi danieli (n = 6) from Swartkrans Member 1. Cercopithecoides williamsi exhibits numerous large pits and puncture pits, and few fine scratches. When diet is considered, C. williamsi is closely associated with C. polykomos and C. apella, suggesting folivorous resources and some hard-objects were consumed, possibly from seeds or grit on fallen fruit, but more likely from grit adhering to the underground storage organs of C4 plants given its mixed C3/C4 isotopic signal. This fossil taxon groups with terrestrial primates more than arboreal ones when habitat is considered. Individuals attributed to C. williamsi largely cluster as a group and a comparison of multiple use-wear scars differentiates open or partially open habitats from forested ones, suggesting dental microwear provides a strong habitat signal.