Acute primary peritonitis in the absence of other comorbid conditions such as liver cirrhosis, immunosuppression, or nephrotic syndrome is a rare disorder in young adults. In women, ascending genital infections are thought to be a major pathogenic cause of this type of peritonitis. Pus was detected in the peritoneal cavity by abdominal paracentesis in a 27-year-old woman who had no predisposing features for severe peritonitis. Abdominal computed tomography showed perirectal edema. Laparotomy was performed, but no intra-abdominal focus of infection could be detected. The abdomen was irrigated via a subhepatic and retroperitoneal presacral approach, and broad-spectrum antibiotic therapy was started. Blood cultures revealed group A streptococci, usually a common cause of upper respiratory tract infections or erysipelas. Within a few days, the patient recovered completely and returned to normal life.