An association between autoimmune pancreatitis (AIP) and inflammatory abdominal aortic aneurysm (AAA) has never been reported. Reported herein is a case of IgG4-related inflammatory AAA accompanying metachronous AIP. A 77-year-old man presented with malaise and intermittent lower abdominal pain. Radiological examination showed inflammatory AAA and right hydronephrosis caused by retroperitoneal fibrosis. Surgical correction of the AAA was performed, but high levels of systemic inflammatory markers persisted. Four months after surgery, the patient presented with epigastric pain, backache, and jaundice. His serum IgG4 concentration was high (571 mg/mL), and he was diagnosed with AIP, based on clinical and radiological findings. Corticosteroid therapy resulted in improvement of the clinical findings and lowered his serum IgG4 levels. Subsequent histological examination of a specimen from the aortic wall showed irregular proliferation of fibroblastic and myofibroblastic cells, severe lymphoplasmacytic infiltration, and obliterative phlebitis in the adventitia. Furthermore, on immunohistochemistry many plasma cells within the lesion were found to be positive for IgG4. These findings suggest that inflammatory AAA has a pathological process similar to that of AIP, and that some cases of inflammatory AAA and retroperitoneal fibrosis may be aortic and periaortic lesions of an IgG4-related sclerosing disease.