Pulmonary hypertension (PH) is distal pulmonary arterial remodelling and is mainly due to the abnormal proliferation and apoptosis resistance of pulmonary artery smooth muscle cells (PASMCs). Apigenin, a natural dietary flavonoid, is a promising PH preventive agent that inhibits PASMC proliferation and induces apoptosis. In this study, we investigated the biological effects of apigenin on PH. PH was induced in male Sprague-Dawley rats by chronic hypoxia exposure. Administration of apigenin prevented the development of PH, hypoxia-induced right ventricular hypertrophy and pulmonary arterial remodelling and prevented the progression of established PH in this model. Moreover, treatment with apigenin induced mitochondria-dependent apoptosis. To explore the underlying mechanisms, the mitochondrial membrane potential (Delta psi m) and the mitochondria-dependent apoptosis factors cytochrome C, BAX, Bcl-2, cleaved caspase 3, and cleaved caspase 9 were analysed. These results confirmed that apigenin induces mitochondria-dependent apoptosis in hypoxic PASMCs to protect against PH. In addition, treatment with apigenin reversed hypoxia-induced inhibition of KV1.5 expression both in vivo and in vitro. The KV1.5 inhibitor diphenyl phosphine oxide-1 (DPO-1) abrogated apigenin-induced mitochondria-dependent apoptosis in hypoxic PASMCs, suggesting that KV1.5 is implicated in apigenin-induced mitochondria-dependent apoptosis. Furthermore, functional studies revealed that apigenin activated mitochondria-dependent apoptosis by modulation of hypoxia-induced factor 1 alpha (HIF-1 alpha) signalling. Together, our study shows that apigenin attenuates PH via inhibiting the HIF-1 alpha-KV1.5 channel pathway to induce PASMC mitochondria-dependent apoptosis.