Necrotizing soft tissue infections (NSTIs) are polymicrobial infections with high morbidity and mortality. We report a case of retroperitoneal NSTI in an immunocompetent young male, with clinical symptoms and signs mimicking acute pancreatitis (pseudo-Cullen’s and pseudo-Grey-Turner’s signs. The initiating lesion was a pustule which progressed and mimics acute pancreatitis. CT scan showed features of retroperitoneal NSTI. After extensive debridement and antibiotics, the patient improved. NSTI of the retroperitoneum is rare. The absence of external clinical signs and anatomical barriers make the disease difficult to manage.