Uterine anomalies result from the failure of complete fusion of the Müllerian ducts during embryogenesis. A unicornuate uterus with a rudimentary horn is the rarest anomaly and results from the failure of one of the Müllerian ducts to develop completely and an incomplete fusion with the contralateral side. Diagnosis and surgical management of a 5-week ectopic pregnancy in a non-communicating rudimentary horn in an 18-year-old nulliparous woman in whom this congenital uterine anomaly was previously unknown are described.