We report a case of giant malignant gastrointestinal stromal tumor (GIST) accompanying an advanced gastric adenocarcinoma. A 73-year-old male patient was admitted to our hospital due to epigastric discomfort. In gastrofiberscopic examination, a localized Borrmann III gastric cancer at the lower body and antrum was noted. In endoscopic ultrasonographic examination, T3 hyperechoic advanced gastric cancer lesion and a relatively well-marginated heterogenous hypoechoic huge mass with a size of 10 cm were noted. In abdomen CT findings, localized wall thickening in the gastric antrum and the anterior wall, and a 11-cm-sized large heterogeneously enhancing mass in gastric body, posterior wall were noted. We did a radical subtotal gastrectomy, including a huge mass, with D2 lymph node dissection. Pathologic findings revealed double primary gastric neoplasms (synchronous occurrence of an adenocarcinoma and a huge GIST). Although closely juxtaposed, these two tumors had not merged and were separated by the thin rim of the muscularis propria.