Silicon's sensitivity to corrosion has hindered its use in photoanode applications. We found that deposition of a ~2-nanometer nickel film on n-type silicon (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochemical (PEC) water oxidation in both aqueous potassium hydroxide (KOH, pH = 14) and aqueous borate buffer (pH = 9.5) solutions. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for oxygen evolution. In 1 M aqueous KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (~1.07 volts versus reversible hydrogen electrode), high photocurrent density, and durability. The electrode showed no sign of decay after ~80 hours of continuous PEC water oxidation in a mixed lithium borate-potassium borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor-like device with an ultrathin, incomplete screening Ni/NiO(x) layer from the electrolyte.