We report a case of a large gastric gastrointestinal stromal tumor (GIST), which became resectable and achieved pathological complete response after neoadjuvant chemotherapy with imatinib mesylate. A 59-year-old man presented with left hypochondrial pain. Abdominal computed tomography (CT) revealed gastric GIST invading the spleen and the diaphragm. Administration of imatinib mesylate was initiated as neoadjuvant chemotherapy. Six months after neoadjuvant chemotherapy with imatinib mesylate, abdominal CT revealed a reduction in tumor size. We judged the tumor resectable and performed partial gastrectomy and splenectomy. Histologically, number of myofibroblasts increased, but no viable tumor cells were observed. Pathological complete response was obtained.