A 21-year-old woman presented gastrointestinal manifestation showing intermittent abdominal pain, nausea, and vomiting. An upper endoscopic examination showed round, elevated mucosa at the antrum of the stomach anterior wall. After CT scanning, a huge degenerated gastrointestinal stromal tumor was suspected. Subtotal gastrectomy with Billroth II anastomosis was performed. Histologically, pseudocystic degeneration of the heterotopic pancreas was confirmed. The patient showed eventful postoperative course except temporary dilated gastric emptying. The patient is doing well without any abnormal symptom at 8-month follow-up. This report is a rare case of gastric outlet obstruction caused by a pseudocyst originating from a heterotopic pancreas in the gastric antrum.