Much effort has been devoted to the investigation of the physical and chemical properties of the Au-O system over a range of pressures, owing to the considerable importance of these materials in fundamental and practical applications. To date, however, only Au1+, Au2+, Au3+, and Au5+ oxidation states have been identified in the Au-O system, but tetravalent Au4+ has not been found. Here, we report the results of structure prediction for the Au-O system at high pressure via the effective structure prediction methodology within a first-principles electronic structure framework. We have uncovered an intriguing structure with AuO2 composition and tetravalent Au, stable at high pressures. This phase shows an electronic transition from a metal to a semiconducting phase as a function of pressure. The present results provide fundamental understanding of the structural and physicochemical properties of compressed Au-O compounds.