Background A retained surgical sponge, also known as a gossypiboma, is a rare cause of serious postoperative complications. Diverse retained surgical materials including instruments such as clamps and sutures have been reported, but surgical sponges are the most common material. We report an unusual case of a gossypiboma mimicking a complicated urachal cyst that led to perforation of the umbilicus. Case presentation A 38-year-old female patient presented in our facility with a palpable periumbilical mass and discharge of pus from the umbilicus for 7 months after an open appendectomy. Since the onset of symptoms, the patient had been treated conservatively in a peripheral hospital where she had been operated on. As no improvement was seen, an ultrasound scan was performed that suggested an intraperitoneal abscess adjacent to the umbilicus. Consequently, the patient was referred to our specialist outpatient department for surgical intervention. Suspecting a complicated urachal cyst, an exploratory laparotomy was performed but revealed a retained surgical sponge as the underlying cause. The gossypiboma was resected, and the postoperative period was unremarkable. Conclusion This case demonstrates that gossypibomas, even though rare, continue to occur. They may clinically and radiologically mimic other pathologies, especially abscesses and tumors. Preventive measures as well as the inclusion of gossypibomas in the differential diagnosis of intraabdominal masses or fistulation detected in patients with a history of surgery are of utmost importance to minimize morbidity, mortality, and potential medicolegal implications.