Introduction: Dermoid cysts are the most common ovarian tumors found during reproductive life. The majority of dermoid cysts are asymptomatic; symptoms appear because of large size and pressure effect, infection, torsion, and rupture of the cyst wall. The present case describes a case in which a dermoid cyst ruptured in a young woman, resulting in chemical peritonitis, which is a rare complication. Case description: The patient was 25 years old and presented with abdominal pain for 2 days and fever for 1 day. Examination revealed mild pallor, tenderness, and guarding in the right quadrant of the abdomen. Her ultrasound and computed tomography (CT) scan suggested a right dermoid cyst with an inflamed omentum and a normal appendix. Emergency laparotomy for possible ruptured dermoid cysts and peritonitis was planned. The sebum was present on the cyst wall and in the peritoneal cavity. Dermoid cystectomy and generous peritoneal irrigation were performed. She recovered well and was discharged on the 3rd postoperative day. Conclusion: Acute abdomen patients need a multidisciplinary team approach to reach a definitive diagnosis. Chemical peritonitis due to rupture of the dermoid cyst requires prompt operative treatment to prevent further complications. Clinical significance: Despite the thick wall of a dermoid cyst, rupture can occur as a rare complication, and patients can present with features of acute abdomen. Prompt diagnosis and management of ruptured dermoid cysts can prevent future complications such as abscess and intestinal obstruction due to adhesions and chronic pelvic pain.