Background <p>Adult T-cell leukemia/lymphoma (ATLL) is an aggressive T-cell neoplasm caused by the human T-cell lymphotropic virus type 1 (HTLV-1). ATLL may present with nonspecific gastrointestinal (GI) symptoms that mimic inflammatory bowel disease (IBD).</p> Case Presentation <p>A 65-year-old man presented with chronic diarrhea, significant weight loss, oral aphthous ulcers, and a 6-month history of lymphadenopathy. Initial laboratory tests showed anemia, and elevated lactate dehydrogenase. Endoscopic findings, including multiple deep ulcers and nodular mucosal changes in the stomach, duodenum, ileum, and colon, initially suggested Crohn’s disease. However, immunohistochemical (IHC) analysis of biopsy specimens from lymph nodes and GI mucosa, combined with HTLV-1 seropositivity, confirmed a diagnosis of high-grade ATLL. Despite combination chemotherapy, the patient’s clinical course was complicated by severe bleeding and airway obstruction, finally leading to death.</p> Discussion <p>This case underscores the diagnostic challenges of differentiating ATLL from IBD based on clinical and endoscopic presentations in HTLV-1 endemic regions. IHC and serological tests are essential for establishing the diagnosis.</p> Conclusions <p>Although rare, ATLL can clinically and endoscopically mimic Crohn’s disease. A high index of suspicion is required when evaluating patients with nonspecific GI symptoms in HTLV-1–endemic areas.</p>