Pulmonary arterial hypertension (PAH) is a severe and progressive disease that leads to right heart failure. The pathogenesis of PAH is generally characterized by vasoconstriction, upregulated proliferation, migration, and pulmonary vascular remodeling in lung tissue. Recent studies using genetic analyses and experimental models have suggested that the hypercontraction of pulmonary arteries induced by Ca²⁺ signaling abnormality may be involved in the pathogenesis of PAH. The Na⁺/Ca²⁺ exchanger type-1 (NCX1) is a bidirectional transporter that is controlled by membrane potential and transmembrane gradients of Na⁺ and Ca²⁺. We recently showed that the upregulation of vascular smooth muscle NCX1 contributes to the development of hypoxia-induced PAH, using NCX1 knockout mice and specific NCX1 inhibitor SEA0400. In the present study, we investigated the pathological role of mitochondrial Na⁺/Ca²⁺ exchanger (NCLX). NCLX knockout mice exhibited significant reduction in right ventricular systolic pressure compared with wild-type mice. Furthermore, specific NCLX inhibitor CGP-37157 significantly suppressed hypoxia-induced PAH in wild-type mice. These findings indicate that NCLX contributes to the development of hypoxia-induced PAH, suggesting that NCLX inhibition might be a novel approach for the treatment of PAH.