Pembrolizumab is an immune checkpoint inhibitor that acts via PD-1 blockade. Recent studies have shown its effectiveness in treating various solid organ tumours. However, unlike cytotoxic chemotherapeutic agents, pembrolizumab may cause immune-related adverse effects. These immune-related adverse effects are generally mild, although patients who experience grade-three or higher side effects may require hospitalisation. In particular, cardiopulmonary side effects are associated with high mortality rates. We report the case of a 24-year-old female patient with alveolar soft part sarcoma accompanied by rare and difficult-to-treat pulmonary hypertension induced by pembrolizumab.