A case of acute appendicitis complicated with necrotizing soft tissue infections (NSTIs) in an extremely elderly woman (98 years-old) is reported. She was admitted to our hospital with a history of increasing pain localized in the right lower abdomen. Abdominal ultrasonography revealed appendicolithiasis and periappendiceal fluid. An appendectomy and closure of the perforated cecum was performed. On the 5th post-operative day, the patient developed skin erythemas and crepitation in the right lower quadrant. An exploration and drainage of the recent operative incision was performed under the diagnosis of NSTIs. Despite the declining overall incidence of appendicitis, it has been increasing among the elderly. The elderly patients are associated with underlying defects in immune function. NSTIs, which are characterized by rapidly progressing inflammation and necrosis of soft tissue, comprise a spectrum of disease ranging from necrosis of the skin to life-threatening infections. The most common etiology of NSTIs was post-operative infections of the abdominal wall, which primarily occurred after operations with extensive fecal contamination. NSTIs are no longer a rare post-operative complication in the elderly and initial treatment should be selected according to the condition of the patient.