A 78-year-old man was diagnosed with adult T-cell leukemia/lymphoma (ATL) and was started on standard chemotherapy 1 year earlier. However, treatment was discontinued because of adverse drug reactions and worsening delirium. Thereafter, he remained stable and was followed up while receiving etoposide. He then visited our hospital because of acute abdominal pain and underwent surgery with a diagnosis of gastrointestinal perforation. Intraoperative observation showed a reddened, concentric wall thickening measuring 4 cm and a perforation site in the ileum, and thus partial resection of the small bowel was performed. The histological diagnosis was small bowel perforation due to tumor cell invasion. Two months postoperatively, he started a less intensive chemotherapy regimen along with palliative care, and died due to the primary disease approximately 5 months postoperatively. ATL involves systemic organs because of its high organ-affinity. Once it involves the gastrointestinal tract, various gastrointestinal symptoms can occur. Patients with ATL are at risk of developing gastrointestinal perforation at any time during the clinical course. Therefore, clinicians should be aware that once gastrointestinal perforation develops, the prognosis becomes extremely poor. Assessment of the disease state, early detection of gastrointestinal lesions, and prevention of opportunistic infections appear to be important in the management of patients with ATL.