We report 2 rare cases of IgG4-related plasma cell granuloma of the maxillary sinus. Histologically, both lesions were characterized by severe lymphoplasmacytic infiltration. In 1 case, fibrous sclerosis and obliterative arteritis were observed. Immunohistochemical study demonstrated that IgG4-positive cells comprised more than 40% of the IgG-positive plasma cells. Moreover, the serum IgG4 level was elevated in both cases. A good responsiveness to steroid therapy has been seen in IgG4-related disease. From a therapeutic perspective, it is important to recognize IgG4-related plasma cell granuloma.