<p>Mature cystic teratomas are common benign ovarian neoplasms, but rupture is a rare complication, occurring in less than 5% of cases. Peritonitis with granulomatous lesions following rupture, mimicking peritoneal carcinomatosis, is extremely rare and is not typically reported as a complication. We report the case of a 48-year-old Caucasian female with lower abdominal pain and suspected four-quadrant peritonitis. A 7.9&#xa0;cm teratoma was identified on CT scan. Exploratory laparoscopy revealed numerous adhesions and fibrinous exudate. Initially, cytoreductive surgery and hyperthermic intraperitoneal chemotherapy were recommended due to a highly suspected malignancy, initially thought to be pseudomyxoma peritonei. However, laparotomy revealed over 50 encapsulated mucoid micronodular lesions and pronounced adhesions throughout the entire abdominal cavity. Despite the macroscopic appearance, no evidence of malignancy was found in any of the numerous frozen sections sent for analysis. Histopathology confirmed a mature cystic teratoma with multifocal foreign body granulomas. No malignancy was detected. It is likely that multiple (silent) ruptures of the mature cystic teratoma were responsible for this unusually pronounced inflammatory response in the whole abdomen. This case report presents a rare and unusual complication of a mature cystic teratoma, where it mimics peritoneal carcinomatosis, leading to an initial misdiagnosis. While ruptured mature cystic teratomas are known, the granulomatous inflammation and peritonitis that mimicked pseudomyxoma peritonei have not been reported in the literature. This novel presentation could significantly affect diagnostic protocols in similar cases, as it provides insight into how granulomatous reactions can present similarly to malignancy, potentially leading to unnecessary aggressive treatments such as cytoreductive surgery and hyperthermic intraperitoneal chemotherapy.</p>