We describe a case of mucosal malignant melanoma in the sinonasal area of a 65-year-old woman. She presented with a history of nasal obstruction and epistaxis with subsequent tenderness, facial anesthesia involving cranial nerve V2, red eye, proptosis, diplopia, and conjunctival chemosis. Computed tomography detected a nonspecific solid mass that had involved the left maxillary sinus and surrounding tissues, with extension into the nasal cavity and invasion of the orbital floor and eye muscles. Histopathologic examination of the neoplasm revealed that it was a malignant melanoma. We performed a radical hemimaxillectomy that extended to the orbit, which allowed for radical excision of the tumor. Postoperatively, the patient received adjuvant chemotherapy and radiotherapy. Mucosal melanoma in the head and neck is a rare and highly malignant neoplasm. We suggest that malignant melanoma be suspected when a small-round-cell tumor is found on light microscopy, and we confirm the usefulness of immunohistochemical investigations.