A 54-year-old woman presented with cardiac metastasis of a Merkel cell carcinoma. Chemotherapy was not effective for the metastasis sites; therefore, radiotherapy was performed for the metastatic cardiac tumors, and it reduced the volume of the cardiac tumors. Cardiac metastasis from Merkel cell carcinoma is rare. Radiotherapy for metastatic cardiac tumors from Merkel cell carcinoma is useful as palliative treatment when the response to chemotherapy is poor.