Among the diverse clinical presentations of gastrointestinal stromal tumor (GIST), spontaneous rupture with peritonitis is extremely rare. We report herein the unusual case of a 75-year-old man found to have a spontaneously ruptured gastric stromal tumor after presenting with generalized peritonitis. The patient was brought to the emergency department of our hospital by ambulance, with generalized severe abdominal pain. On examination, his abdomen was extensively distended with generalized severe rebound tenderness. Abdominal computed tomography scan showed a giant mass arising from the anterior gastric wall with an irregular internal low-density area and a small amount of ascites. An emergency laparotomy revealed a ruptured gastric tumor with dissemination of its necrotic tissue throughout the peritoneal cavity. The tumor was excised together with normal gastric tissue around its base. The tumor, which was 15 x 11 x 4.4cm in size, had a coarse laceration over its well-capsulated smooth serosal surface with massive necrosis and clotted blood inside. Immunohistochemical examination revealed positive reactivity to C-kit protein, which was consistent with the newly introduced diagnostic criteria of GIST. The patient had an uneventful postoperative course and remains well.