IgG4-related disease of the stomach is a rare disorder, and only a few cases have been reported. We present two cases that were identified over a 2-month period in our center. Two male patients aged 52 and 48 years presented with mass lesion in the stomach, which were clinically thought to be gastrointestinal stromal tumor, and they underwent excision of the lesion. Microscopic examination revealed marked fibrosis, which was storiform in one case, associated with diffuse lymphoplasmacytic infiltration and an increase in IgG4-positive plasma cells on immunohistochemistry. Serum IgG4 level was markedly elevated. Although rare, IgG4-related disease should be considered in the differential diagnosis of gastric submucosal mass lesions.