Abstract. The current study describes a case of adenosquamous carcinoma originating from a thyroglossal duct cyst (TGDC). A 77-year-old man presented with an asymptomatic mass in the left mid-neck, which was soft and mobile on palpation. Fine-needle aspiration was performed, but cytology did not detect any malignant cells. Computed tomography demonstrated a single cystic lesion in the left lobe of the thyroid gland; therefore, surgery was performed on the suspected thyroid cyst. However, it was identified intraoperatively that the lesion was separated from the thyroid gland and instead adhered to an additional hyoid bone; therefore, the Sistrunk procedure was performed. Histopathological examination of the resected tumor confirmed the diagnosis of adenosquamous carcinoma originating from a TGDC. Carcinoma arising from a TGDC is rare, and accounts for 1% of all TGDC cases. The most common subtype of carcinoma associated with TGDC is papillary carcinoma, whilst adenosquamous carcinoma developing from a TGDC is extremely rare, with only one case currently reported in the literature. Although a consensus for the management of this disease has not yet been established, adequate surgical excision with long-term follow-up is currently the preferred treatment.