Histiocytic sarcoma is a rare lymphohematopoietic malignancy with aggressive clinical course and poor therapy response. The diagnosis relies on the confirmation of its histiocytic lineage and exclusion of other poorly differentiated tumors. Most of the cases present in extranodal sites, but primary gastric involvement is exceptional. We report a case of a 69-year-old woman with epigastric pain and systemic symptoms. Gastroscopy findings and biopsy report suggested a malignant neoplasm. The patient underwent distal subtotal gastrectomy with a 6-cm tumor in the body and antrum of the stomach and ten associated enlarged perigastric lymph nodes. Microscopically they were infiltrated with atypical tumor cells and immunohistochemical staining was positive for CD68, lysozyme, CD45, and CD4; 45% of the cells stained for Ki-67. The pathologic diagnosis was histiocytic sarcoma. CT body scans showed only enlarged retroperitoneal and abdominal lymph nodes. The patient received six cycles of CHOEP chemotherapy with complete therapeutic response, but three months later she experienced an aggressive systemic sarcoma recurrence and although salvage chemotherapy was initiated she died of progressive disease. The presented case widens the differential diagnosis of gastric malignancies, and emphasizes the significance of immunohistochemical examination for histiocytic sarcoma diagnosis. The collection and evaluation of cases of gastric histiocytic sarcoma are important to obtain further progress in prognosis and treatment.