Disorders of lung development may result in secondary pulmonary arterial hypertension (PAH), a complex condition marked by elevated pulmonary arterial pressure and impaired function of the right ventricle. Yes-associated protein (YAP), a pivotal mediator of the Hippo pathway, has gained growing recognition as a vital component in the signaling network that regulates cell behavior during lung development. However, the roles of Yap in both alveolar development and vascularization as well as its involvement in the pathogenesis of PAH remain unknown. By investigating mice with a conditional deletion of YAP specifically in alveolar epithelial type 1 (AT1) cells, we demonstrate that Yap deficiency results in impaired alveolar development and vascularization, which aligns with the exacerbation of hypoxia-induced PAH. These pathological changes occur concomitant with compromised endothelial growth factor A (VEGFA) secretion, a known angiogenic factor required for alveolar development and lung angiogenesis. Rescue experiments confirmed that VEGFA acts as a downstream target of Yap and plays a crucial role in both alveolar development and vascularization. Taken together, these data bear significant implications for comprehending the intricate mechanisms governing alveolar development and pulmonary vascular remodeling, thereby holding great promise for advancing our understanding of Group 3 PAH.