The aim of this case report is to present the root canal treatment of a maxillary left lateral incisor fused to a supernumerary tooth associated with incomplete root development. After thorough chemomechanical debridement of the root canal using 3% sodium hypochlorite and 2% chlorhexidine, aqueous-based calcium hydroxide was used as an intracanal dressing. The apical 3-4 mm of the canal were sealed with mineral trioxide aggregate. The 1-year follow-up radiograph revealed successful apical closure and periapical healing. This case report points out that: (i) A careful clinical and radiographic examination is necessary to establish the diagnosis of fusion with a supernumerary tooth or with a normal tooth. (ii) Even in a fused tooth with an open apex and associated with large periapical radiolucency, careful nonsurgical root canal treatment might result in satisfactory periradicular healing. (iii) The single-visit MTA plug method is an alternative to multi-visit apexification in the treatment of immature teeth with open apex. Keywords: developmental anomalies, fusion, gemination, MTA, open apex