Abstract. Appendicitis in the neonatal period is extremely rare. Its low incidence together with non-specific clinical symptoms often mean the diagnosis is delayed, leading to increased rates of peritonitis and mortality. We report the case of a 33-week premature infant, small for gestational age (1180 g at birth), clinically stable and receiving exclusive enteral feeding, who presented clinical manifestations of necrotizing enterocolitis at 14 days of life. Acute phase reactants were elevated and abdominal radiography showed pneumoperitoneum. Laparotomy revealed acute perforated appendicitis without intestinal involvement and purulent fluid in the peritoneum, for which appendectomy was performed. Neonatal acute appendicitis should be considered in the differential diagnosis of abdominal sepsis since early diagnosis and treatment significantly reduce associated morbidity and mortality.