Tolvaptan is an oral vasopressin V2-receptor antagonist recognized as effective for fluid retention associated with congestive heart failure and liver cirrhosis. However, there have been no reports concerning clinical experience with tolvaptan for sinusoidal obstruction syndrome (SOS). A 42-year-old male with primarily refractory T-lymphoblastic lymphoma underwent allogeneic peripheral blood stem cell transplantation from an HLA-matched sibling donor. The myeloablative conditioning regimen consisted of busulfan and cyclophosphamide. On day 20, the total bilirubin level was elevated to 2.0 mg/dL, and body weight increased from 76 to 85 kg, allowing a diagnosis of SOS to be made. Treatments with thrombomodulin, furosemide, carperitide, and low-dose dopamine were ineffective. By day 27, the patient's body weight had increased to 90 kg, and he subsequently developed cardiopulmonary failure. Therefore, we administered low-dose tolvaptan for 2 days (3.75 mg on day 27 and 7.5 mg on day 28). Consequently, his ascites and edema were significantly reduced, and body weight returned to 77 kg by day 34. However, he died of lymphoma progression on day 55. Tolvaptan may be an alternative and promising treatment for refractory fluid retention associated with SOS, although it is unclear whether tolvaptan administration leads to improvement in clinical outcome.