Chronic contained rupture of an abdominal aortic aneurysm (AAA) is a rare event that is difficult to diagnose due to the atypical and chronic symptoms. We report a case of chronic contained rupture of AAA mimicking a retroperitoneal tumor in a 36-year-old man. The patient presented with weight loss and chronic lower abdominal pain, and was referred to our clinic with a suspected retroperitoneal tumor. Abdominal computed tomography (CT) revealed a distinct mass measuring 15 x 10 x 10 cm in the left retroperitoneal space, involving the abdominal aorta. One week later he experienced sudden abdominal pain radiating to the back. He was subsequently diagnosed with ruptured AAA and aortic dissection. The patient received implantations of both common iliac arteries to the abdominal aorta using Y-grafts and an ascending-to-descending aortic graft in a two-stage operation. His recovery from surgery was uneventful.