Dens invaginatus in molar teeth is extremely rare. This report presents a case in a right mandibular second molar. The pulp was necrotic and there was a buccal sinus tract associated with the tooth. The dens invaginatus was revealed following a periapical radiograph that also showed a periradicular lesion. Conventional root canal treatment was carried out; a calcium hydroxide paste was used as an intracanal dressing. The root canal space was filled with gutta-percha and sealer using a thermomechanical compaction technique. At follow-up, after 1 year, the tooth was symptom-free and there was radiographic evidence of healing. As a result, endodontic surgery was not required. Keywords: dens invaginatus, molar dens invaginatus, tooth abnormality