OBJECTIVE The aim of this study was to compare the mesiodistal tooth sizes and to see the gender dimorphism among three ethnic groups of southern part of Saudi Arabia, that is, Asir, Najran, and Jizan region. METHODS The sample composed of 90 individuals (45 of each gender) from 3 different regions of southern Saudi Arabia, having mean age of 26.9 years. The measurements were done directly on the models with the aid of the digital calliper measuring the largest mesiodistal measure of incisors, canines, premolars, and molars in both sides. RESULTS Almost all the mesiodistal tooth widths were statistically significant between the groups except maxillary and mandibular permanent third molars. Regarding sexual dimorphism, the groups were combined; almost all the mesiodistal tooth widths were not shown statistically significant difference between the males and females except that right first permanent premolars were significantly larger in the females than in the males. CONCLUSIONS The present study has provided norms of the mesiodistal permanent tooth diameters of a Saudi population that are newer and more complete than the previously published norms. There appear to be some secular trends in the mesiodistal diameter of the Saudi permanent teeth toward sexual dimorphism.