Adult T-cell leukemia/lymphoma (ATLL) is a rare malignancy caused by human T-lymphotropic virus type 1 (HTLV-1). ATLL arising after liver transplantation is extremely uncommon, and all previously reported cases only involve recipients transplanted for acute liver failure. We report a 69-year-old man who developed acute-type ATLL 15 years after living-donor liver transplantation (LDLT) for hepatocellular carcinoma —representing the first such case in the literature and the longest latency described. He presented with gastrointestinal symptoms and leukocytosis with 32% atypical lymphocytes. Workup revealed HTLV-1 positivity, hypermetabolic lymphadenopathy, and bone marrow involvement. After the diagnosis of acute type ATLL, he received VCAP-AMP-VECP chemotherapy and intrathecal therapy, followed by interferon-α and allogeneic stem cell transplantation. Despite treatment, he developed severe complications and died 11 months after diagnosis. This case expands the clinical spectrum of post-transplant ATLL, demonstrating that disease may occur many years after transplantation and even in recipients transplanted for HCC rather than acute liver failure. We propose a risk-stratified surveillance strategy for HTLV-1 carriers after liver transplantation.