Hypoxia-inducible factor-1 (HIF-1) is a transcription factor involved in the cancer cell adaptation to hypoxia, a leading cause of tumor malignancy. Thus, control of HIF-1α expression may assist in treatment of cancer. The expression of HIF-1α is finely regulated via alterations in not only HIF-1α protein stability but also mRNA stability. However, the molecular mechanisms of regulation of HIF-1α mRNA stability have not yet been fully elucidated. Here, we show that tristetraprolin (TTP) protein, of which the mRNA expression level is downregulated in most of hepatocellular carcinoma tissues, bound directly to the 3′-UTR of HIF-1α mRNA containing eight putative TTP-binding motifs, AU-rich elements (AUUUA), to downregulate stability. Furthermore, TTP expression was induced in hypoxic cells, and overexpression of TTP repressed the hypoxic induction of HIF-1α protein. Taken together, these data suggest that TTP is a modulator of HIF-1α expression during hypoxia and may play a physiological role in regulation between cellular adaptation and apoptosis in prolonged hypoxia. In addition, cancer cells may benefit from the downregulation of TTP, which subsequently increases HIF-1α expression and assists with the adaptation of cancer cells to hypoxia.