The purpose of this paper was to describe the case of an 11-year-old patient who presented a dens invaginatus, detected in the permanent maxillary left lateral incisor, with an immature open apex and extensive apical periodontitis and sinus tract. The mineralized invaginated barrier was removed, and a nonsurgical root canal treatment was performed using both calcium hydroxide as a root canal dressing to stimulate apexification and a mineral trioxide aggregate (MTA) plug to permit root canal filling. After a 14-month period of root canal dressing changes, no evidence of apical periodontitis was observed, and the MTA plug was placed in the root canal's apical portion. The root canal filling was performed using the thermoplasticized gutta-percha technique. At the 12-month follow-up, complete radiographic periapical healing, characterized by bone formation in the area around the tooth apex, and no clinical or radiographic evidence of refractory apical periodontitis were detected.