Necrotizing fasciitis is a potentially life-threatening infection that can lead to rapid muscular and fascial necrosis, often resulting in sepsis. In addition to the rapid disease progression, diagnosing this disease in children can be challenging as they cannot accurately communicate their symptoms. Spontaneous necrotizing fasciitis secondary to Clostridial infection has rarely been described in the literature but occurs in neutropenic patients with significant morbidity and mortality from myonecrosis and gas gangrene. This case report describes fulminant nontraumatic necrotizing fasciitis in a 15-month-old child receiving chemotherapy for acute myeloid leukemia. The child had risk factors, including neutropenia, immunosuppression, and prior antibiotic use, which possibly contributed to Clostridial colonization of the gastrointestinal tract and eventual hematogenous spread. He rapidly developed progressive swelling and erythema of the right thigh after venous phlebotomy. Imaging revealed subcutaneous emphysema with evidence of necrotizing fasciitis and deep compartmental collections. The disease spread rapidly, involving the scrotum and lower abdominal wall. Despite parenteral antibiotics and extensive surgical debridement, the patient's condition deteriorated rapidly, with refractory septic shock and its sequelae, and he eventually died. This case study highlights the critical importance of prompt response to any suspicion of necrotizing fasciitis in immunocompromised pediatric patients to prevent life-threatening complications.