Cercopithecoides williamsi, a Plio-Pleistocene primate, is believed to have been a terrestrial colobine monkey. Dental microwear analysis of C. williamsi specimens from South African cave sites was employed to test these assumptions. Analysis of the features shows that although the microwear signature of C. williamsi is similar to that of folivorous primates, there are also similarities with terrestrial papionins. Overall, the dental microwear analysis demonstrates that C. williamsi could have indeed been a folivorous, terrestrial monkey. A high amount of puncture pits also points to a substantial amount of grit in the diet. Similarities between the microwear features of C. williamsi and Cebus apella indicate that fruit or hard objects could have been a supplemental food of C. williamsi. The consumption of underground storage organs covered in grit would explain the heavy pitting of C. williamsi teeth. Being terrestrial, C. williamsi would have been in direct competition with terrestrial papionins.