We performed sliding transport distraction osteogenesis (STDO) of an alveolar segment containing an unerupted third molar in the mandible of a 22-year-old man with a benign cementoblastoma. Marginal mandibulectomy including the tumour and the right mandibular second premolar and first and second molars was done. STDO was performed to horizontally reconstruct the alveolar ridge and to restore occlusion with the use of the third molar. After forward horizontal distraction of the alveolar segment, the third molar spontaneously erupted and was gradually moved to the position previously occupied by the second molar.