Malignant melanoma is a neoplasm of melanin-producing cells predominantly of cutaneous origin, which uncommonly develops within gut mucosa. We present the case of a 58-year-old woman with complaints of abdominal pain, loss of appetite and weight.Esophagogastroduodenoscopy revealed a gastric mass and systemic imaging demonstrated widespread nodal and bilateral adrenal gland involvement. Histopathology of the gastric mass confirmed primary malignant mucosal melanoma of the stomach. The patient received three cycles of Nivolumab but did not respond, and thus, was then offered best supportive care. Although infrequent, mucosal melanoma can arise from the gastrointestinal tract, and in contrast to the cutaneous form, advanced disease usually has a dismal prognosis and responds poorly to immune checkpoint inhibitors. Primary gastric melanoma is an aggressive disease that is diagnosed by exclusion after the differential diagnosis of metastasis from a cutaneous or unknown primary site has been conducted. If available, patients with treatment-naïve mucosal melanoma should be considered for enrollment in clinical trials.