The goal of tooth autotransplantation is to restore a missing tooth by using a functional tooth from the same patient. Although this procedure is gaining more recognition as a viable treatment option in dentistry, it is important to monitor and report long-term results. This case report presents the long-term follow-up of a fused maxillary central incisor that was extracted and replaced with a supernumerary tooth. Although the donor tooth displayed favorable characteristics, including an intact periodontal ligament and minimal extraoral time, which contributed to the success of the transplantation, it also presented certain disadvantages, such as having a fully developed root apex. Over a nine-year follow-up period, no clinical symptoms indicative of functional failure were observed. However, external cervical root resorption, first detected at the six-year follow-up, progressed by the ninth year. Despite the lack of clinical symptoms, radiographic findings suggested that the root resorption was a significant concern. This case underscores the potential for long-term success of autotransplanted teeth, even in complex scenarios involving fused teeth. However, it also highlights the importance of thorough, long-term monitoring to detect complications such as root resorption. The findings suggest that, while autotransplantation can offer favorable outcomes, careful management and periodic evaluations are essential to ensure the longevity of the transplanted tooth.