Dermoid cyst and epidermoid cyst develop due to inflammation or fetal trauma, and dermoid cyst aberrantly include epithelial tissue. However, the relationship between the pathological tissue and lesion image in these diseases is unclear. Here, we report a rare case of a patient in whom a large dermoid cyst was incidentally detected on the floor of the mouth during treatment of a dentofacial deformity. The presence of the cyst was confirmed by computed tomography (CT) and histological examination. The 16-year-old female patient presented with a swelling extending from the floor of the mouth to the chin. On initial examination, a painless, elastic, soft swelling of the mouth floor was observed, but tongue movement, articulation, and eating were not impaired. CT revealed an oval, well-defined, cystic lesion approximately 60 × 40 × 32 mm in size. Intraoral cystectomy was performed under general anesthesia. At the 1-year follow-up, there was no sign of recurrence or occlusion. The patient’s facial profile improved after postoperative orthodontic treatment.