The rudimentary horn of uterus is an extremely rare malformation and potentially serious obstetric entity, threatening maternal and fetal outcome. Diagnostic sonography of early pregnancy in a non-communicating rudimentary horn is difficult but important. We report a case of ruptured non-communicating rudimentary horn with unicornuate uterus at 12 weeks' gestation, where diagnosis is made before surgery. Excision of the rudimentary horn and ipsilateral salpingectomy (to prevent a further ectopic tubal gestation), conserving the ovary, is the recommended surgical procedure for patients desiring to maintain their fertility potential. The subsequent obstetric prognosis is reassuring. Diagnostic imaging examinations of the reproductive system after this treatment showed no negative effect from surgery on subsequent fertility and there was no reported case of uterine rupture during subsequent pregnancy in the remaining unicornuate uterus after rudimentary horn excision. Future pregnancies will require extremely close monitoring and a caesarean section is highly recommended.