A 74-year-old woman with refractory adult T-cell leukemia/lymphoma (ATLL) received three courses of mogamulizumab. Despite obtaining complete remission, she thereafter presented with progressive ascites. An analysis of the ascites and laboratory tests revealed no evidence of ATLL invasion, infectious disease, or liver cirrhosis. The mogamulizumab concentrations were maintained in the ascites at approximately 10-15% of that in the plasma. Mogamulizumab was considered to be a plausible pathogenesis of her ascites. To the best of our knowledge, this is the first report suggesting mogamulizumab-induced ascites.