This case report describes the treatment of a dens invaginatus in a mandibular premolar with large periradicular lesion. A nonsurgical endodontic treatment was performed to tooth #28. During the treatment procedure, calcium hydroxide and a triple antibiotic paste were separately used for intracanal dressing. The apical third of the canal was obturated with mineral trioxide aggregate (MTA), and the remaining canal space was obturated with gutta-percha and AH 26 root canal sealer, using a lateral condensation technique. At follow-up examination after 1 year, the tooth was asymptomatic and radiographically showed repair of the lesion. Healing was achieved without any need for further endodontic surgical intervention.