Lymphedema is a chronic edema that sometimes occurs after treatment of gynecologic cancer, and cellulitis often occurs concomitantly with lymphedema. On the other hand, necrotizing fasciitis (NF) is a relatively rare, but life-threatening disease. The symptoms in cellulitis and NF are very similar. In this case report, we describe a case in which the diagnosis of NF in a lymphedematous limb was difficult. A 70-year-old woman had secondary lymphedema in bilateral legs and consulted our department. On the first day of lymphedema therapy, the patient complained of vomiting, diarrhea, and fever (37.7 °C) without local fever in the legs. She was diagnosed with acute gastroenteritis. On the next day, swelling and pain in her left leg occurred and her blood pressure was 59/44 mmHg. She was diagnosed with cellulitis accompanied by lower limb lymphedema and septic shock. On the second day, blisters appeared on the left leg, and computed tomography showed NF. We performed debridement under general anesthesia and her vital signs improved postoperatively. Streptococcus agalactiae (B) was detected in blood culture, and we administered bixillin and clindamycin. Postoperatively, necrosis in the skin and fat around the left ankle gradually spread, and it took 5 months to complete epithelialization. The diagnosis was more difficult than usual NF because patients with lymphedema often experience cellulitis. Clinicians should always think of NF to avoid mortality due to delayed treatment. This case report was approved by the institutional ethics committee.