Introduction: Identification of possible anatomical variations is crucial for the correct diagnosis and planning of endodontic therapy. Objective: To report the endodontic treatment of tooth #37, in which a third canal was located in the mesial root (middle mesial canal - MMC). Methods: Tooth #37 was diagnosed with symptomatic irreversible pulpitis and referred for endodontic treatment. Coronary cracks were found during coronal access using a dental operating microscope (DOM). A cone-beam computed tomography (CBCT) was requested. CBCT revealed cracks involving the pulp chamber floor and a third root canal in the mesial root, between the mesiobuccal and the mesiolingual canals. Endodontic treatment was performed with the WaveOne GOLD reciprocating system. The chemical-mechanical preparation was performed with 5.25% sodium hypochlorite and 17% EDTA to remove the smear layer, followed by another irrigation with sodium hypochlorite. The canals were dried with absorbent paper points. The root canal obturation was performed with the continuous wave of condensation technique. Results: The root canals of tooth #37 were correctly instrumented and obturated. The patient did not present with any postoperative symptoms, and was referred for final prosthetic rehabilitation. Conclusion: CBCT and DOM were paramount for the correct planning of the case, as regards the presence of cracks and the detection of the middle mesial canal.