INTRODUCTION: Retroperitoneal necrotizing fasciitis is a rare, fulminant, and potentially lethal complication of intra-abdominal suppuration. A retroperitoneal origin is very rare and very few cases have been reported in the literature. To the best of our knowledge, this case is only the fourth case reported of successful management following retroperitoneal necrotizing fasciitis. CASE PRESENTATION: A 33-year-old Tamang man presented to our facility with a history of five days of fever and vomiting and eight days of severe left loin pain. On examination, he had features of peritonism. A laparotomy was performed, revealing extensive necrotizing fasciitis of the retroperitoneum extending to the anterior abdominal wall. Our patient survived following extensive debridement of the necrotic tissues and supportive care. CONCLUSIONS: Retroperitoneal necrotizing fasciitis can rarely present with features of peritonism, and hence should be included as a possible differential diagnosis for anyone presenting with peritonism. Although a fatal condition, early intervention and aggressive management can save the life of a patient.