Background: A dermoid cyst is a cystic lesion of developmental origin and uncertain etiology that rarely affects the floor of the mouth. It usually presents as a non-tender, slow-growing swelling in the sublingual region. Case presentation: A large lesion was found in the sublingual and submandibular regions of a 15-yearold female patient who presented to our clinic with the chief complaint of a sublingual mass that had been present for many years but had enlarged over the last year. Computed tomography (CT) scans revealed a giant cystic mass that was a 4.6 x 3.3 cm flask-shaped cystic lesion with a non-enhancing wall in the midline of the sublingual space. Magnetic resonance imaging (MRI) with contrast revealed a well-defined cystic mass with heterogeneous fluid on all sequences. The histopathology report was suggestive of a dermoid cyst. The intraoral approach was preferred for surgery, and no recurrence or complaints were detected during the follow-up period. Conclusion: The pathological diagnosis of the presented case was an epidermoid cyst. For giant dermoid cysts, a surgical incision is considered an effective treatment of choice. This offers a good prognosis and very low chances of recurrence. To achieve the best cosmetic effects, an intraoral technique should be chosen whenever possible.