We report a case of primary gastric malignant melanoma that was diagnosed after curative resection but initially misdiagnosed as adenocarcinoma. A 68-year-old woman was referred to our department for surgery for gastric adenocarcinoma presenting as a polypoid lesion with central ulceration located in the upper body of the stomach. The preoperative diagnosis was confirmed by endoscopic biopsy. We performed laparoscopic total gastrectomy, and the final pathologic evaluation led to the diagnosis of primary gastric malignant melanoma without a primary lesion detected in the body. To the best of our knowledge, primary gastric malignant melanoma is extremely rare, and this is the first case reported in our country. According to the literature, it has aggressive biologic activity compared with adenocarcinoma, and curative resection is the only promising treatment strategy. In our case, the patient received an early diagnosis and underwent curative gastrectomy with radical lymphadenectomy, and no recurrence was noted for about two years.