Abstract Body (Do not enter title and authors here): Pulmonary arterial hypertension (PAH) is marked by progressive vascular remodeling, but key cell type–specific regulatory programs driving this process remain poorly defined. We performed single-nucleus RNAseq of 67 PAH and donor lungs to generate the largest cell atlas of the human PAH lung to date, capturing all major vascular, immune, and stromal subpopulations. Myofibroblasts were significantly expanded in PAH and enriched for epithelial-to-mesenchymal transition and PAH-associated genetic variants. Ligand–receptor interaction and latent factor analysis identified a signaling program enriched for endothelial–stromal crosstalk and TGFβ signaling, correlating with histologic measures of remodeling. Trajectory analysis revealed that adventitial fibroblasts had the highest developmental potential among stromal cells. In a focused trajectory analysis modeling their transition to myofibroblasts, ITGA9 emerged as a top driver gene, with expression correlating with clinical severity. Spatial transcriptomics confirmed ITGA9 expression in myofibroblasts within remodeled vessels. ChIPseq demonstrated SMAD3 and SP1 binding at the ITGA9 promoter, supporting transcriptional regulation by TGFβ–related pathways. ITGA9 was elevated in PAH across independent datasets, including bulk lung tissue, animal models, and peripheral blood. Regulatory variants linked to ITGA9 expression in myofibroblasts also correlated with disease severity. These findings from an unbiased atlas-scale single-cell analysis of the PAH lung identify ITGA9 as a transcriptionally regulated driver of fibroblast remodeling, and nominate it as a potential biomarker and therapeutic target in PAH.