Anatomic variations of the pulmonary artery (PA) cause vascular injuries and result in critical mistakes. Here we report the first case of lung cancer with a fissureless left upper lobectomy, an aberrant mediastinal trunk of the lingular and basal segments of the PA. A 65-year-old man was referred to our hospital with a solid mass on the left upper lobe. A fissureless left upper lobectomy was performed due to severe incomplete lobulation. Intraoperative findings showed an extremely rare anatomic variation (left A5+A8+A9b) that arose as a common trunk from the left main PA. To prevent intraoperative injury, it is essential to consider the unexpected mediastinal inferior branch and perform a surgical procedure such as fissureless lobectomy upon encountering incomplete lobulation.