Mogamulizumab (Mog) and lenalidomide (Len) are new therapeutic candidates for relapsed adult T‐cell leukemia/lymphoma after allogeneic hematopoietic stem cell transplantation (allo‐HSCT). In the present study, we retrospectively analyzed 12 patients who received Mog or Len monotherapy for relapsed ATL after allo‐HSCT. Eight and three patients received Mog and Len, respectively. The remaining patient received Mog for the first relapse and Len for the third relapse. A complete response was achieved by three and two patients who received Mog and Len, respectively, two and one of whom remained alive with a complete response for more than 20 months. In terms of adverse events, the emergence or progression of graft‐versus‐host disease was observed in three out of four patients treated with Len and in none of the patients treated with Mog. The development or progression of cytomegalovirus reactivation was detected in four out of eight patients treated with Mog and in none of those treated with Len. The present results suggest that Mog and Len would be promising treatment options for relapsed ATL after allo‐HSCT and need to be selected based on adverse event profiles.