Accessory cavitated uterine mass (ACUM) is a rare Müllerian anomaly presenting as pelvic pain in young women, often misdiagnosed as ovarian pathology. We report a 21-year-old female presenting with right lower abdominal pain. Ultrasound suggested a complex ovarian cyst. Magnetic resonance imaging (MRI) demonstrated a well-defined, non-communicating intramyometrial cavitary lesion with hemorrhagic contents and normal ovaries. Laparoscopic excision confirmed the diagnosis. MRI is vital to prevent inappropriate surgical intervention.