We present a case of adult T-cell leukemia (ATL) with jejunal perforation at the site of intestinal involvement by ATL. A 39-year-old woman presented with sudden-onset abdominal pain. Physical examination showed generalized severe abdominal tenderness and intraabdominal free air was seen on radiographic examination. Under a diagnosis of peritonitis due to intestinal perforation, an emergency operation was performed. A pinhole-like perforation was found in the jejunum 80 cm distal to Treitz's ligament, and the patient underwent partial resection of the affected jejunum. Microscopic examination revealed diffuse infiltration of abnormal lymphocytes into the entire wall of the jejunum and mesenteric lymph nodes. A diagnosis of ATL was confirmed by the presence of antibody to human T-lymphotropic virus type 1 (HTLV-1) in the serum, a positive result for T-cell markers and the HTLV-1 proviral genome in the mononuclear cells in the specimens. The final diagnosis was thus lymphoma subtype of ATL. Combination chemotherapy was repeated until the patient died 14 months postoperatively. Emergent surgery followed by intense chemotherapy might improve survival in patients with ATL and perforated intestine.