Fournier gangrene is a rapidly progressing necrotizing fasciitis involving the perineal, perianal, or genital regions and constitutes a true surgical emergency with a potentially high mortality rate. Although the diagnosis of Fournier gangrene is often made clinically, emergency computed tomography (CT) can lead to early diagnosis with accurate assessment of disease extent. CT not only helps evaluate the perineal structures that can become involved by Fournier gangrene, but also helps assess the retroperitoneum, to which the disease can spread. Findings at CT include asymmetric fascial thickening, subcutaneous emphysema, fluid collections, and abscess formation. Subcutaneous emphysema is the hallmark of Fournier gangrene but is not seen in all cases. Compared with radiography and ultrasonography, CT provides a higher specificity for the diagnosis of Fournier gangrene and superior evaluation of disease extent; however, diagnosis and evaluation can also be performed with these other modalities. The administration of broad-spectrum antibiotics and aggressive surgical debridement of the nonviable tissue are both essential for successful treatment. An awareness of the CT features of Fournier gangrene is imperative for prompt diagnosis and effective treatment planning.