Sarcomatoid carcinoma is an uncommon biphasic malignant tumor of the stomach. The histogenesis remains unknown, and a definitive diagnosis is obtained with immunohistochemical staining. Since prognosis is poor after surgery, more effective diagnostic tools are needed in order to select the optimal therapeutic approach. We report the case of an old female patient presenting with an endophytic tumor in the gastric fundus who underwent partial resection of the stomach. Histology revealed a poorly differentiated adenocarcinoma component mixed with sarcomatoid component. Immunohistochemically, the carcinoma components exhibited a positive reaction to pan-cytokeratin, whereas fusiform cells showed positive reactions to vimentin.