To survive in hypoxic environments, organisms must be able to cope with redox imbalance and oxygen deficiency. The SIRT1 deacetylase and the HIF-1α transcription factor act as redox and oxygen sensors, respectively. Here, we found that SIRT1 binds to HIF-1α and deacetylates it at Lys674, which is acetylated by PCAF. By doing so, SIRT1 inactivated HIF-1α by blocking p300 recruitment and consequently repressed HIF-1 target genes. During hypoxia, SIRT1 was downregulated due to decreased NAD⁺ levels, which allowed the acetylation and activation of HIF-1α. Conversely, when the redox change was attenuated by blocking glycolysis, SIRT1 was upregulated, leading to the deacetylation and inactivation of HIF-1α even in hypoxia. In addition, we confirmed the SIRT1-HIF-1α interaction in hypoxic mouse tissues and observed in vivo that SIRT1 has negative effects on tumor growth and angiogenesis. Our results suggest that crosstalk between oxygen- and redox-responsive signal transducers occurs through the SIRT1-HIF-1α interaction.