Hypoxic pulmonary hypertension (HPH) is characterized by active vasoconstriction and profound vascular remodeling. KLF5, a zinc-finger transcription factor, is involved in the excessive proliferation and apoptotic resistance phenotype associated with monocrotaline-induced pulmonary hypertension. However, the molecular mechanisms of KLF5-mediated pathogenesis of HPH are largely undefined. Adult male Sprague-Dawley rats were exposed to normoxia or hypoxia (10% O-2) for 4 wk. Hypoxic rats developed pulmonary arterial remodeling and right ventricular hypertrophy with significantly increased right ventricular systolic pressure. The levels of KLF5 and hypoxia-inducible factor-1 alpha (HIF-1 alpha) were upregulated in distal pulmonary arterial smooth muscle from hypoxic rats. The knockdown of KLF5 via short-hairpin RNA attenuated chronic hypoxia-induced hemodynamic and histological changes in rats. The silencing of either KLF5 or HIF-1 alpha prevented hypoxia-induced (5%) proliferation and migration and promoted apoptosis in human pulmonary artery smooth muscle cells. KLF5 was immunoprecipitated with HIF-1 alpha under hypoxia and acted as an upstream regulator of HIF-1 alpha. The cell cycle regulators cyclin B1 and cyclin D1 and apoptosis-related proteins including bax, bcl-2, survivin, caspase-3, and caspase-9, were involved in the regulation of KLF5/HIF-1 alpha-mediated cell survival. This study demonstrated that KLF5 plays a crucial role in hypoxia-induced vascular remodeling in an HIF-1 alpha-dependent manner and provided a better understanding of the pathogenesis of HPH.