Necrotizing fasciitis rarely involves the retroperitoneum, and their mortality is quite high. The diagnosis is difficult due to the structure of the retroperitoneal region, and the imaging tool with the highest sensitivity is computed tomography (CT). A 57-year-old male patient was admitted to the emergency department for abdominal pain, fever, and urinary retention. Acute phase reactants were elevated in laboratory values. Urinary ultrasound and abdominal CT were performed on the patient with the suspicion of acute pyelonephritis. Non-enhanced abdominal CT revealed diffuse gas and contamination of fatty tissue in the left retroperitoneal area. He was taken to urgent surgery with the diagnosis of retroperitoneal necrotizing fasciitis. Necrotizing fasciitis most commonly occurs in the extremities and perianal region and is clinically aggressive. Patients usually have an underlying factor such as immunosuppression or diabetes mellitus. CT may show severe stranding of fatty tissue, thickening of the fascia, and abscess. This infection, which has high morbidity and mortality, should be diagnosed quickly and debridement should be performed urgently.