T-cell posttransplant lymphoproliferative disorders after solid-organ transplant are rare and may be clinically aggressive. A 3-year-old boy had liver transplant from his grandfather because of hepatoblastoma. The immunosuppressive regimen was based on tacrolimus and prednisolone. At 22 months after transplant (age, 5 years), the patient presented to the hospital because of severe cough. Computed tomography scan of the chest showed a large left mediastinal mass (9 × 7.2 × 7 cm) and left pleural effusion. A Tru-Cut biopsy of the mediastinal mass showed diffuse infiltration with blast cells, and the diagnosis of T-cell acute lymphoblastic leukemia was made. Immunohistochemical examination of blasts showed strong and diffuse terminal deoxynucleotidyl transferase and CD3 antibody expression; Ki-67 proliferation index was > 95%, and tumor cells were negative for Epstein-Barr virus. Tacrolimus was stopped, sirolimus was started, and chemotherapy was given, but he died 2 months after diagnosis because of chemotherapy-induced sepsis. Monomorphic T-cell posttransplant lymphoproliferative disorder with features of acute lymphoblastic leukemia and lymphoblastic lymphoma is rare after liver transplant.