This case report describes a woman with spontaneous rectal perforation from decidualized endometriosis in pregnancy. A 37-year-old woman was admitted to our hospital at 30 wk of pregnancy with symptoms suggestive of pyelonephritis, which persisted until 33 wk of gestation when delivery of a premature male baby was performed through a cesarean section.On postoperative day 2, an abdominal computed tomography showed free air in the peritoneal cavity and a pelvic abscess. Explorative celiotomy revealed a diffuse severe fecaloid peritonitis that originated from a 3-cm wide rectal perforation. A Hartmann operation was then performed. Histopathological findings were consistent with decidualization of the rectal wall. Only 20 cases of intestinal perforation due to endometriosis have been reported in the literature. This report is believed to be the first case of spontaneous rectal perforation from endometriosis in pregnancy, and it shows the potential occurrence of serious and unexpected complications of the disease.