The prognosis for relapsed adult T-cell leukemia/lymphoma (ATL) after allogeneic hematopoietic stem cell transplantation is poor. Here, we report the case of a 67-year-old man who survived for 26 months after treatment with lenalidomide for post-transplant relapsed ATL. He underwent induction therapy with two cycles of modified VCAP-AMP-VECP and achieved complete remission. He received cord blood cell transplantation following a reduced-intensity conditioning regimen. Seven months after transplantation, swelling of the systemic lymph nodes appeared, and relapsed ATL was diagnosed based on a biopsy of the cervical lymph node. Treatment with 10 mg of lenalidomide induced partial remission. At 18 months after transplantation, skin tumors were successfully treated by increasing the dose of lenalidomide to 15 mg with the emergence of skin graft-versus-host disease. Although he died from ATL at 34 months after transplantation, systemic relapsed lesions were controlled by treatment with lenalidomide for 26 months. Our case suggests that lenalidomide is well tolerated and is an effective option for the treatment of post-transplant relapsed ATL.