BACKGROUND We report a case of ruptured ovarian teratoma mimicking pelvic inflammatory disease (PID) and ovarian malignancy. The case indicates the need for reviewing the information on ovarian teratomas, as the symptoms are vague, and, therefore, diagnosis and treatment had to be structured accordingly. METHODS A 60-year-old woman was admitted to the emergency department with acute lower abdominal pain. She experienced weight loss and increased abdominal girth. Pelvic ultrasound and computed tomography revealed a 14-cm pelvic tumor. Laboratory examination revealed leukocytosis (white blood cell count: 12620/μL, segment: 87.7%) and high levels of C-reactive protein (18.2 mg/dL). Elevated levels of the tumor marker cancer antigen 19-9 (367.8 U/mL, normal value < 35 U/mL) were also noted. Due to the impression of a ruptured tubo-ovarian abscess or a tumor with malignancy, she immediately underwent an exploratory laparotomy. A ruptured ovarian tumor with fat balls, hair strands, cartilage, and yellowish fluid was observed on the right side. Right salpingo-oophorectomy was performed. A pathological examination revealed a mature cystic teratoma. The patient recovered after surgery and was discharged on post-operative day three. No antibiotics were administered. CONCLUSIONS This case illustrates the differential diagnosis of an ovarian tumor. Therefore, surgery is the mainstay for treating a ruptured teratoma.