Rationale: Although many familial cases of pulmonary arterial hypertension (PAH) exhibit an autosomal dominant mode of inheritance with the majority having mutations in essential constituents of the bone morphogenetic protein (BMP) signaling, the specific contribution of the long-term loss of signal transduction triggered by the type 2 BMP receptor (BMPR2) remains poorly characterized. Objective: To investigate the role of BMP9, the main ligand of ALK1/BMPR2 heterocomplexes, in pulmonary hypertension (PH). Method and Results: The absence of BMP9 in Bmp9-/-mice and its inhibition in C57BL/6 mice using neutralizing anti-BMP9 antibodies substantially prevent against chronic hypoxia induced PH judged by right ventricular systolic pressure (RVSP) measurement, right ventricular hypertrophy, and pulmonary distal arterial muscularization. In agreement with these observations, we found that the BMP9/BMP10 ligand trap ALK1ECD administered in monocrotaline (MCT) or Sugen/hypoxia (SuHx) rats substantially attenuate proliferation of pulmonary vascular cells, inflammatory cell infiltration and regresses established PH in rats. Our data obtained in human pulmonary endothelial cells derived from controls and PAH patients indicate that BMP9 can affect the balance between endothelin-1, apelin and adrenomedullin. We reproduced these in vitro observations in mice chronically exposed to hypoxia, with Bmp9-/-mice exhibiting lower mRNA levels of the vasoconstrictor peptide endothelin (ET)-1 and higher levels of the two potent vasodilator factors apelin and adrenomedullin (ADM) compared with Bmp9 +/+ littermates. Conclusion: Taken together, our data indicate that the loss of BMP9, by deletion or inhibition, has beneficial effects against PH onset and progression.