This case report presents successful immediate mandibular third molar autotransplantation to replace a non-restorable mandibular first molar. In this case, after the extraction of the non-restorable tooth, the donor molar with incomplete root formation was autotransplanted into the recipient site after atraumatic extraction. A long term continuous follow-up of 2 years revealed that the tooth was fixed in its socket without residual inflammation; masticatory function was satisfactory and without discomfort; the tooth was not mobile; no pathological condition was apparent radiographically; the lamina dura appeared normal and the tooth showed radiographic evidence of root growth. Pulpal regeneration and the depth of the pocket, gingival contour, and gingival colour were all normal. The purpose of this case report is to demonstrate the clinical application of autotransplantation as a future option for permanent restoration without implants, orthodontic space closure or partial denture. Palabras clave: autotransplantation, complete root formation, pulpal regeneration