The extracellular matrix (ECM) increasingly emerges as an active driver in several diseases, including idiopathic pulmonary arterial hypertension (IPAH). The basement membrane (BM) is a specialized class of ECM proteins. In pulmonary arteries, the BM is in close contact and direct proximity to vascular cells including endothelial cells. So far, the role of the BM has remained under-investigated in IPAH. Here, we aimed to shed light on the involvement of the BM in IPAH, by addressing its structure, composition and function. On an ultrastructural level, we observed a marked increase in BM thickness in IPAH pulmonary vessels. BM composition was distinct in small and large vessels and altered in IPAH. Proteoglycans were mostly responsible for distinction between smaller and larger vessels, while BM collagens and laminins were more abundantly expressed in IPAH. Type IV collagen and laminin both strengthened endothelial barrier integrity. However, only type IV collagen concentration dependently increased cell adhesion of both donor and IPAH-derived pulmonary arterial endothelial cells (PAEC), and induced nuclear translocation of mechanosensitive transcriptional co-activator of the hippo pathway Yes-activated protein (YAP). On the other hand, laminin caused cytoplasmic retention of YAP in IPAH PAEC. Accordingly, silencing of COL4A5 and LAMC1, respectively, differentially affected tight junction formation and barrier integrity in both donor and IPAH PAEC. Collectively, our results highlight the importance for a well-maintained BM homeostasis. By linking changes in BM structure and composition to altered endothelial cell function, we here suggest an active involvement of the BM in IPAH pathogenesis.