Primary mediastinal large B-cell lymphoma (PMBCL) is relatively infrequent and generally has a good prognosis with standard immunochemotherapy. However, treatment options are limited for patients with relapsed/refractory PMBCL who are ineligible for stem cell transplantation. In this report, we treated a refractory PMBCL patient, who did not respond to salvage chemotherapy, with combined nivolumab and radiotherapy. The patient achieved a complete remission with mild adverse reactions and has survived without relapse 2 years after treatment.