We herein report an unusual case of an infected descending aortic pseudoaneurysm with luminal pathognomonic oscillating vegetation with serological findings and clinical features mimicking anti-proteinase 3-antineutrophil cytoplasmic antibody-associated vasculitis. The positive blood cultures and imaging findings, including a pseudoaneurysm and vegetations in the aorta, suggested the presence of an infected aortic aneurysm. The patient was successfully treated with antibiotics and endovascular aortic repair. A precise diagnosis is crucial in order to avoid inappropriate therapy such as immunosuppressive treatment, which could result in life-threatening consequences in a patient with an infected aortic aneurysm.