Background: An accessory and cavitated uterine mass (ACUM) is a rare congenital Mullerian anomaly where an accessory cavity with endometrial lining lies attached to the normal functioning uterus. It is located at the level of the insertion of the round ligament. It occurs due to Mullerian duct tissue's duplication or persistence, which originated from gubernaculum dysfunction, leading to accessory uterine tissue formation. It is common among young and nulliparous women presenting severe dysmenorrhea and infertility. It is mostly seen in women below 30 years of age. Rudimentary and uterine horns and degenerating fibroids are the possible differential diagnoses. Case description: A 45-year-old female presented with complaints of pain abdomen and irregular menstrual cycles for 2 months. On magnetic resonance imaging (MRI), a well-defined cystic lesion was seen in the left ovary measuring 54 mm × 40 mm × 36 mm, which was suggestive of a left ovarian benign cystic lesion – hemorrhagic cyst. On Pelvic ultrasound, a complex cyst of 74 mm × 52 mm with a hyperechoic area of 36 x 31 mm suggests a left ovarian tumor. The patient belonged to the perimenopausal age group and completed the family, so a total abdominal hysterectomy with bilateral salpingo–oophorectomy was done and sent for histopathological examination. Conclusion: Transvaginal ultrasonography (TUV) and MRI aid in diagnosis. Early surgical treatment of the accessory cavitary mass is recommended treatment in this case. Detailed histopathological examination is mandatory to meet the criteria for its complete diagnosis. Clinical significance: A rare congenital mullerian is a rare and challenging condition to diagnose. Especially when fertility is desired, a complete medical history, gynecological examination, and radiological analysis are necessary for a correct diagnosis. Laparoscopic excision of the ACUM can be performed in patients where fertility is desired.