Immunoglobulin G4-related sclerosing cholangitis (IgG4-SC) is a rare autoimmune disease characterized by fibroinflammatory lesions and bile duct strictures, often associated with type 1 autoimmune pancreatitis (AIP). Isolated IgG4-SC, occurring without AIP, is particularly uncommon and can clinically and radiologically mimic hilar cholangiocarcinoma, presenting with jaundice and bile duct strictures. Accurate differentiation between these conditions is essential, as surgical resection is the standard treatment for cholangiocarcinoma, whereas steroid therapy is the first-line treatment for IgG4-SC. This case report discusses a 55-year-old female patient who underwent left hepatectomy due to a hilar bile duct stricture initially suspected to be cholangiocarcinoma but was ultimately diagnosed as isolated IgG4-SC based on postoperative histopathological and immunohistochemical findings. The report highlights the diagnostic challenges of isolated IgG4-SC and emphasizes the importance of integrating histology, imaging, and serology to prevent unnecessary surgical interventions.