Neurofibromatosis type 1 is an autosomal dominant disorder with cutaneous features and multiple tumors that involve different organs. Gastrointestinal stromal tumors are non-epithelial tumors associated with neurofibromatosis type 1 and arise from the gastrointestinal tract. Patients with gastrointestinal stromal tumor present most commonly with anemia due to overt or occult bleeding and in rare cases diagnosis might be delayed so patients present with gastrointestinal tract obstruction. Here, we report a rare case of an older patient with neurofibromatosis type 1, who was undiagnosed and presented with acute abdomen due to gastric outlet obstruction. It was caused by progressive gastrointestinal stromal tumor in the antrum of the stomach. Gastrointestinal stromal tumors should be considered in the differential diagnosis of gastric outlet obstruction, especially in patients with neurofibromatosis type 1.