Pulmonary arterial hypertension (PAH) is a rare and severe progressive disorder characterized by high pulmonary artery pressure. Chronic hypoxia causes a metabolic disorder and the Warburg effect in pulmonary arterial smooth muscle cells (PASMCs). Pyruvate dehydrogenase kinase 1 (PDK1) is a key enzyme in Warburg effect increased by hypoxia‐inducible factor (HIF‐1). We constructed a cell‐based luciferase assay system for HIF‐1 inhibitors. Using this system, six HIF‐1 inhibitors were identified. Among these inhibitors, the effect of tagitinin C (1) on PASMC was investigated. Tagitinin C (1) clearly decreased the amount of HIF‐1β and the HIF‐1 target PDK1. This result indicates that HIF‐1 inhibitors effectively decrease PDK1 activity, which is a cause of the metabolic disorder and Warburg effect observed in PASMCs. Identifying naturally occurring HIF‐1 inhibitors could provide novel insights into the development of PAH medications.