Inflammatory myofibroblastic tumor (IMT) is a space occupying lesion which is composed of myofibroblasts, plasma cells, and lymphocytes. IMT of the maxillary sinus is rare and its etiology is unknown. We present a case of inflammatory myofibroblastic tumor occurring in the right maxillary sinus of a 57-year-old woman. Radiologically, this tumor was interpreted as malignant neoplasm. On histologic examination, bundles of spindle cells were admixed with inflammatory cells including mature plasma cells and lymphocytes. On the basis of the immunohistochemical findings and ultrastructural features, we recognized that the intervening spindle cells were myofibroblasts. We discussed etiology and prognostic factors of this tumor.