Abstract Background Alimentary tract duplications are rare anomalies and any delay in the diagnosis may develop various complications. Infection, hemorrhage, or perforation of the duplication cyst may lead to an acute presentation. Occasionally, it may lead to a diagnostic and management dilemma. Herein, we report an unusual complication of gastric duplication cyst. Case presentation A 2.5-year-old girl presented with recurrent abdominal distension, fever, and abdominal pain. The imaging workup revealed a huge intraperitoneal collection. Intraoperatively, a huge pseudocyst was encountered communicating with the gastric duplication cyst. The gastric duplication cyst was sharing a common wall with the greater curvature of the stomach but was not communicating with the gastric lumen. Pseudocyst along with gastric duplication cyst was completely excised. The resultant seromuscular defect of the stomach was also closed. The postoperative period was uneventful. Conclusion Perforation of the gastric duplication cyst should be kept in differentials of intraperitoneal collection not amenable to multiple aspirations. Huge intraperitoneal collection secondary to perforation of gastric duplication is exceedingly rare and scarcely reported in the literature.