We report a case of giant rectal gastrointestinal stromal tumor (GIST) successfully resected by abdominoperineal resection combined with posterior vaginal wall resection. Our patient was a 79-year-old woman had been diagnosed as having von Recklinghausen disease at the age of 30 years. In 2006, a computed tomography (CT) scan revealed a tumor originating from the posterior wall of the rectum. In June 2010, she was admitted to our hospital with the chief complaint of bloody stool. A CT scan revealed a giant tumor that had increased to 9cm in size. Colonoscopy demonstrated a submucosal tumor, which was subsequently diagnosed by biopsy as a GIST. Due to the patient's continuous bleeding and abdominal pain, an emergent abdominoperineal resection was performed at that time. The tumor, which measured 8×9cm, was immunohistochemically positive for c-kit and CD34. Concomitant resection of the posterior wall of the vagina via the perineal approach provided a wide area to expose the tumor for a safe operation. The patient has been well with no sign of recurrence since the operation. This procedure is recommended as a useful approach for giant rectal GISTs or rectal cancer with invasion of the posterior pelvic wall.