Unicornuate uterus with a rudimentary horn is a rare Müllerian anomaly, with an incidence of 0.06%. Due to this relative rarity, an accurate preoperative diagnosis of unicornuate uterus with rudimentary horn still remains to be a diagnostic challenge. Reported here is a case of a 27-year-old nulligravida who complained of severe debilitating dysmenorrhea 5 months prior to admission, necessitating frequent visits to the emergency department for administration of intravenous pain medications and antiemetics. She was misdiagnosed preoperatively with subserous leiomyoma, mainly based on clinical presentation, and findings on ultrasound and pelvic MRI. On exploratory laparotomy, a definitive diagnosis of unicornuate uterus with an enlarged rudimentary horn was established, and patient underwent resection of rudimentary horn, with enterolysis, due to dense adhesions in the left adnexa and cul de sac. We emphasize the need for early diagnosis and prompt management of cases like this to prevent complications, and optimize fertility and quality of life of affected patients.