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Pulmonary arterial hypertension (PAH) is a multifactorial and chronic disease characterized by an increase in pulmonary artery (PA) resistance leading to right ventricle (RV) failure. Endothelial dysfunction and alteration NO/cGMP signaling in PA play a major role in PAH. We described the involvement of Rac1 in the control of systemic blood pressure through its involvement in NO-mediated relaxation of arterial smooth muscle cell (SMC). The aim of this study was to decipher the role of SMC Rac1 in PAH. Rac1 activation, Rac-GTP, is determined by immunofluorescence in lungs biopsies from control and PAH patients. PAH is induced by exposure of wild-type (WT) and SMC Rac1-deficient mice (SM-Rac1-KO mice) to chronic hypoxia (10% O 2 , 4 weeks). PAH is assessed by the measurement of RV systolic pressure and hypertrophy. The proliferation of pulmonary arterial SMC (PASMC) in hypoxic condition is measured by EdU staining. Oxidative stress is assessed by immunofluorescence using the DHE probe and by electron spin resonance (ESR). In biopsies from PAH patients, we detected an increase (3-fold) of Rac-GTP in PA compared to control (n = 4–6). To decipher the role of Rac activation in PAH, WT and SM-Rac1-KO mice were exposed to hypoxia to induce PAH. The elevation of systolic pressure in RV and RV hypertrophy were decreased by 32.5% and 60% (n = 5–7) in SM-Rac1-KO mice. Deletion of Rac1 in SMC reduced the hypoxia-induced remodeling. In vitro, we observed that hypoxia-induced PASMC proliferation is decreased by 50% by the Rac1 inhibitor, EHT1864. We demonstrated that Rac1 inhibition or deletion in SMC prevent ROS production in PASMC and limit PASMC proliferation and vascular remodeling. Our results suggest that Rac1 activity in PASMC plays a causal role in PAH by favoring ROS dependent PA remodelling induced by chronic hypoxia. Therefore, Rac1 deletion or inhibition protect against hypoxic PAH. [ABSTRACT FROM AUTHOR]