We report a very rare case of de novo abscess formation inside an endometrioma without any prior intervention in a 32-year-old woman. She presented with acute abdominal pain and persistent high-grade fever. A lump was palpable and tender and showed features of endometrioma on ultrasound. Laparotomy was conducted, as her clinical condition deteriorated despite broad spectrum antibiotics, and her fever did not subside. Thick foul-smelling pus was drained from the endometrioma, and the abscess wall was removed, followed by cystectomy. The patient improved markedly and did well in the postoperative period.