The existence of a third root in the mandibular first molars may have implications for certain dental treatments; however, its greatest relevance could be in anthropological and forensic sciences, because its prevalence varies significantly according to ethnic groups. The purpose of this research was to determine the prevalence of radix entomolaris (RE) in a sample of Chilean people. A random sample of 1330 digital periapical radiographs of endodontically treated mandibular first molars (551 males and 779 females, aged between 18 and 87 years) were evaluated. The prevalence of RE, its distribution by sex and root canal lengths were determined. Evidence demonstrated that the formation of RE is associated with ancestry and its frequency is variable according to populations studied, being more frequent in Asians and Native Americans. The prevalence of RE in the studied people was 3.16%, similar to frequencies reported in European individuals (3.4%-4.2%). The 61.90% of RE were located at the right side of the mandible and 38.10% at the left side. The average working length of RE was 19.52 mm. No statistically significant differences in frequencies were observed by sex (p = 0.19). The prevalence of RE varies among continental population groups. However, due to human migration, the frequency of this physical trait could be modified in specific geographic regions. The observed prevalence of RE in the study group was similar to the prevalence ranges observed for European ancestry populations and it was considerably lower than the data reported for non-Europeans. The absence of sexual dimorphism is consistent with results of previous studies performed worldwide.