We report a rare case of spontaneous uterine rupture of an unscarred uterus caused by adenomyosis in the early third trimester. A 33-year-old primigravid woman was referred to our department because of severe acute abdominal pain and signs and symptoms of hemorrhagic shock. Ultrasound exanimation performed at admission revealed a living, intrauterine fetus of 28 weeks gestational age with reduced amniotic fluid and presence of free peritoneal fluid. The fetal heart rate was non-reassuring with variable decelerations and severe fetal bradycardia. Emergency cesarean section revealed massive hemoperitoneum and complete rupture in the uterine fundus. Subtotal peripartum hysterectomy with conservation of adnexae was performed. Histological examination revealed adenomyosis at the site of uterine rupture.