Mandibular molars can have an additional root located lingually (the radix entomolaris) or buccally (the radix paramolaris). If present, an awareness and understanding of this unusual root and its root canal morphology can contribute to the successful outcome of root canal treatment. The purpose of this study was to report a clinical case of successful endodontic treatment in mandibular molar characterized as radix entomolaris. Key learning points • Clinicians should be aware of this unusual root morphology in mandibular first molars in Caucasian people. • Radiographs exposed at two different horizontal angles are needed to identify this additional root. • The access cavity must be modified in a distolingual direction in order to visualize and treat the RE, this results in a trapezoidal access cavity.