Fournier's gangrene (FG) is an atypical, life-threatening polymicrobial infection characterized by the rapid destruction of soft tissue, predominantly in the perineal region. Retroperitoneal spread of FG represents an uncommon condition described in a few case reports, and its presentation as the first manifestation of the human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) is even more infrequent. Here, we present the case of a 40-year-old male who was admitted to the emergency department with a lowgrade fever of 37.8°C, abdominal pain, and four-day history of sharp, bilateral testicular pain and swelling. On physical examination, the patient was hypotensive with necrotic tissue in the perineum. A computed tomography study displayed an extensive retroperitoneal spread of suspected FG. Due to the massive spread of the infection, an HIV test was requested, yielding positive results, which indicated that HIV/AIDS had first manifested as FG with retroperitoneal extension. This is an extremely rare initial presentation of HIV/AIDS. To treat the patient and address the severe necrosis, a peritoneal lavage, surgical debridement, right orchiectomy, and colostomy were performed. After the procedure, antiretroviral therapy was established with tenofovir, emtricitabine, and efavirenz.