BACKGROUND: Aberrant BMPR2 (bone morphogenetic protein receptor 2) signaling is associated with the pathogenesis of pulmonary hypertension. SMAD4 (mothers against decapentaplegic homolog 4) is an essential downstream effector of BMPR2 signaling, but whether and how it participates in pulmonary hypertension are unclear. METHODS: Globally and vascular cell-specifically inducible knockout mouse models were used to examine the role of SMAD4 deficiency in differential cell compartments in the development of pulmonary hypertension. Single-cell transcriptomic analysis in combination with in vitro cell function measurements was conducted to provide mechanistic insights into pulmonary hypertension pathogenesis. RESULTS: Adult mice with Smad4 global deletion or endothelial cell-specific deletion spontaneously developed pulmonary hypertension manifestations, characterized by elevated right ventricle systolic pressure and excessive muscularization in pulmonary distal vessels, which were accompanie