Merkel cell carcinoma is a rare and aggressive neuroendocrine-derived skin cancer arising most commonly on the sun-exposed head and neck skin of elderly and immunocompromised patients. Although a combination of wide excision and adjuvant radiotherapy is the optimal therapeutic approach for Merkel cell carcinoma, radiation monotherapy has recently been recommended for unresectable tumors. We report here a case of Merkel cell carcinoma treated with radiation monotherapy and reviewed Merkel cell carcinoma cases treated with radiotherapy alone in Japan. A 75-year-old man was referred for treatment of a tumor on the upper lip with a swollen submental lymph node. The histopathological diagnosis from biopsied material was Merkel cell carcinoma (T3N1bM0, stage IIIB). The submental lymph node was extirpated and radiation monotherapy was applied according to the 2014 National Comprehensive Cancer Network Guidelines because the Eastern Cooperative Oncology Group Performance Status of the patient was grade 3 and the patient and his family did not desire surgery. The primary site and bilateral upper neck regions were irradiated with 45 Gy followed by 20 Gy irradiation for the primary site alone. Three months after radiotherapy, the tumor seemed to have completely remitted. Approximately 1 year after radiotherapy, no evidence of local recurrence or late metastasis has been noted. Radiation monotherapy should be considered as a curative treatment for Merkel cell carcinoma, particularly in situations where extensive surgery is not favored.