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A ruptured rudimentary horn pregnancy is an obstetric emergency. It presents clinically similar to a ruptured tubal ectopic pregnancy. In patients with a known uterine anomaly, it is a risk that can be anticipated; however, in patients with a previous caesarean, who are unaware of having a uterine anomaly, it becomes a diagnostic challenge. We present such a case of a multigravida with a previous caesarean section who presented at 15 weeks gestation with severe abdominal pain and hypotension. On ultrasonography, a single extrauterine fetal demise with features of uterine rupture and massive haemoperitoneum was noted. Subsequently, exploratory laparotomy revealed a unicornuate uterus with a left-sided ruptured non-communicating horn. This case highlights the complexity of the diagnosis and management of acute abdomen in pregnancy with features of shock in an undiagnosed case of unicornuate uterus. It also emphasises the importance of documentation and communication of intraoperative findings with the patient.