The hypoxia inducible factor (HIF) pathway has been considered to be an attractive anti-cancer target. One strategy to inhibit HIF activity is through the disruption of the HIF-1 alpha-p300 protein-protein interaction. We report herein the identification of an osmium(II) complex as the first metal-based inhibitor of the HIF-1 alpha-p300 interaction. We evaluated the effect of complex 1 on HIF-1 alpha signaling pathway in vitro and in cellulo by using the dual luciferase reporter assay, co-immunoprecipitation assay, and immunoblot assay. Complex 1 exhibited a dose-dependent inhibition of HRE-driven luciferase activity, with an IC50 value of 1.22 mu M. Complex 1 interfered with the HIF-1 alpha-p300 interaction as revealed by a dose-dependent reduction of p300 co-precipitated with HIF-1 alpha as the concentration of complex 1 was increased. Complex 1 repressed the phosphorylation of SRC, AKT and STAT3, and had no discernible effect on the activity of NF-kappa B. We anticipate that complex 1 could be utilized as a promising scaffold for the further development of more potent HIF-1 alpha inhibitors for anti-cancer treatment.