Granulocytic sarcoma, also called chloroma or myeloblastoma, is an extramedullary invasive tumor composed of neoplastic myeloid cells. In this report, we describe a 43-year-old male patient with a collision tumor composed of an adenocarcinoma and a granulocytic sarcoma in the stomach. The coexistence of a granulocytic sarcoma and adenocarcinoma in the stomach has, to the best of our knowledge, not been reported in the literature. The diagnosis of granulocytic sarcoma is very difficult; especially in the absence of concurrent hematologic disease or in the uncommon setting of coexistence with another tumor. Cautious observation is needed when a finding of unusual atypical cells admixed with an adenocarcinoma in the stomach is confronted.