Abstract: Bone morphogenetic proteins 9 (BMP9), a member of bone morphogenetic protein (BMPs), belongs to the transforming growth factor (TGF-β) family and exerts a broad range of biological functions. It plays a critical role in regulating endothelial cell function and maintaining vascular homeostasis. Recent studies have found that BMP9 is closely related to lung diseases, and its signaling pathway plays an important role in the development of lung diseases. This paper briefly reviews BMP9 and its signal transduction process, as well as its involvement in pulmonary hypertension, pulmonary fibrosis, lung cancer and other lung diseases.