We report herein a case of leiomyosarcoma of Meckel's diverticulum which presented as a rare manifestation of perforation. A previously healthy 63-year-old man was referred to the Tetsujinkai Eniwa hospital following the sudden development of acute abdominal pain. Abdominal computed tomography revealed a solid mass in the pelvic cavity, and an emergency operation was performed under a provisional diagnosis of peritonitis associated with a pelvic tumor. A perforated tumoral mass was found in Meckel's diverticulum. Segmental resection of the ileum, including the tumor-bearing diverticulum, was performed along with regional lymph node dissection. Histologic examination revealed the lesion to be leiomyosarcoma.