Fusobacterium necrophorum infection is known to cause Lemierre's syndrome, not pelvic peritonitis. Herein, we report a case of Fusobacterium necrophorum pelvic peritonitis and bacteremia, without Lemierre's syndrome, mimicking intestinal necrosis. A 28-year-old woman with peritoneal irritation and shock was suspected of having intestinal necrosis due to the presence of hepatoportal venous gas and pneumatosis intestinalis. Intestinal necrosis was ruled out by emergency laparotomy. However, massive opaque ascites and inflammatory changes in the uterus and fallopian tubes were observed. Fusobacterium necrophorum and Gardnerella vaginalis were found in ascetic fluid cultures. Moreover, Fusobacterium necrophorum was also found in blood culture. Systemic management of septic shock and antibiotic treatment improved the patient's general condition and abnormal gas on imaging. The patient had untreated bacterial vaginosis prior to admission. Pelvic peritonitis caused by Fusobacterium necrophorum is extremely rare. However, it must be recognized to avoid its rapid development into severe onset mimicking intestinal necrosis.