Infected aneurysms caused by Listeria monocytogenes are extremely rare. Therefore, there is no standard procedure for their diagnosis and treatment. A 76-year-old Japanese man with diabetes and hypertension was diagnosed with a left common iliac aneurysm caused by L. monocytogenes, using multidetector computed tomographic angiography and rapid diagnostic testing of the positive blood culture. He was successfully treated with a combination of ampicillin administration, timely surgical debridement, and in-situ Y-graft placement with revascularization and omental implantation. Vancomycin and third-generation cephalosporins, to which L. monocytogenes is resistant, are used as an empirical regimen for infected aneurysms. Therefore, the use of a rapid diagnostic testing is important as it identifies L. monocytogenes within 24 h from obtaining the blood cultures, and guides the administration of the appropriate antibiotics. In-situ Y-graft placement restores nearly normal blood flow, following the confirmation of negative conversion of blood culture in response to the intensive intravenous ampicillin therapy. Appropriate and timely microbiological examinations, in addition to radiographic examinations, can be the key for selecting the optimal therapeutic procedures for each patient and achieving the best possible outcomes.