We report a case of autoimmune pancreatitis involving the colonic mucosa. Although serum level of IgG4 was normal, computed tomography and endoscopic retrograde cholangiopancreatography showed diffuse enlargement of the pancreas and irregular narrowing of the pancreatic ducts, respectively. Colonoscopy revealed a polypoidal lesion in the ascending colon. A lymphoplasmacytic infiltration was seen both in the pancreas and in the polypoidal lesion of the colon. Furthermore, immunohistochemical analysis showed abundant IgG4-positive plasma cells in these lesions. This is the first case report of a simultaneous occurrence of autoimmune pancreatitis and a colonic polypoidal lesion, both of which are characterized with increased IgG4 responses.