Necrotizing fasciitis is a rare disease characterized by rapidly progressive soft tissue infection primarily involving the superficial fascia and is associated with significant morbidity and mortality. Necrotizing fasciitis of the genitalia and the perineum, also known as Fournier's gangrene, usually occurs after local trauma, perirectal or perineal infections, and complicated surgery, such as circumcision and herniorraphy. The lack of initial external clinical signs, because the process begins in the deep subcutaneous tissue, make early diagnosis and adequate surgical management difficult. The progression of the disease is often fulminant, and the prognosis hinges on accurate diagnosis and immediate surgical debridement. The present case report documents the rare development of fulminant necrotizing fasciitis associated with a rectal cancer surgery and radiation therapy.