Mogamulizumab induces cytotoxicity against CCR4+ lymphoma cells by antibody-dependent cell-mediated cytotoxicity in advanced cutaneous T-cell lymphoma patients. Since the efficacy of mogamulizumab in mycosis fungoides (28.6%) is lower than that in Sézary syndrome (47.1%), reagents that enhance the antitumor immune response induced by mogamulizumab are needed to further optimize its use for the treatment of erythrodermic mycosis fungoides. In this report, we present a case of erythrodermic mycosis fungoides successfully treated with mogamulizumab followed by etoposide monotherapy.