Histiocytic sarcoma is a rare malignant neoplasm that originates from a histiocytic hematopoietic lineage characterized by histiocytic differentiation and its corresponding immunophenotypic features. Patients with histiocytic sarcoma usually have a poor prognosis due to its aggressive clinical behavior. Here we report a rare case of extranodal histiocytic sarcoma of the stomach which was confirmed through immunohistochemical staining. A 71-year- old man was presented with epigastric pain. Gastroscopy, abdominal CT, and EUS revealed a mass located on the posterior wall of upper body and fundus of the stomach. Grossly, grayish white solid masses were seen extending down to the submucosal layer. Microscopically, the tumor cells had eosinophilic cytoplasm, abundant vacuole, and mitosis. Immunohistochemical staining revealed that the tumor cells were positive for LCA, CD68, and lysozyme. Early detection and accurate diagnosis of this rare neoplasm is important because it can make a great difference in prognostic outcomes. To make an accurate and definitive diagnosis, immunohistochemical staining is essential in the confirmation of histiocytic origin.