The number of canals, as well as the existence and form of roots, are most variable in mandibular first molars. Most first mandibular molars have a single mesial and distal root, but occasionally, a mandibular molar termed as a Radix entomolaris (RE) will show evidence of an additional lingual root located distally. To ensure successful treatment outcomes, it is essential to diagnose, identify and treat these variations with a comprehensive understanding of root and root canal architecture and configurations. This case report details the endodontic treatment of a mandibular first molar exhibiting RE.