Here, we present the case of ruptured intercostal artery pseudoaneurysm two months after blunt thoracic trauma. Ruptured aneurysm hemorrhaged into the retroperitoneal space due to adhesion in the intrathoracic space. We unsuccessfully embolized the aneurysm during an angiographic procedure and performed the aneurysmectomy, removing the hematoma, using a retroperitoneal approach. Ruptured intercostal artery pseudoaneurysm after blunt thoracic trauma may have caused the delayed life-threatening hemorrhage.