Electrode potential–pH (Pourbaix) diagrams provide a phase map of the most stable compounds of a metal, its corrosion products, and associated ions in solution. The utility of these phase diagrams is that they enable the assessment of electrochemical stabilities, for example, of Ni metal and its derived oxides, hydroxides, and oxyhydroxides, against corrosion in aqueous environments. Remarkably, the Ni Pourbaix diagrams reported over the last 50 years are largely inconsistent with various electrochemical observations, which may be attributed to inaccurate experimental free energies of formation (Δf G) for the complex Ni-based compounds used in producing the available diagrams. Here we show that state-of-the-art density-functional theory (DFT) can be used to obtain accurate Δf G values, which lead to Ni Pourbaix diagrams that are more consistent with direct electrochemical experiments: Electrochemical impedance spectroscopy and surface-enhanced Raman spectroscopy are used to characterize the electrochemical stabilities of NiO and Ni­(OH)2 formed on Ni, demonstrating the reliability in correction-free first-principles based Pourbaix diagrams. Our results show the importance in applying modern density functionals in combination with experimental advances in aqueous environment compound identification for assessing electrochemical phase stability of materials, which will be useful for the design, synthesis, and selection of corrosion-resistant metals, photoabsorbers, and photocatalytic materials.