Chemical modification of BiVO₄ nanoparticles (Scheelite, E_G = 2.62 eV) with chemically deposited Co₃O₄ nanoparticles improves the photocatalytic water oxidation activity by a factor of 17 to 11 mmol g⁻¹ h⁻¹ under visible light (AQE 10% at 435 nm) and to 1.24 mmol g⁻¹ h⁻¹ under sunlight from aqueous 0.02 M NaIO₄. This activity ranks among the highest among known visible light driven water oxidation photocatalysts. Based on systematic electrochemical, photoelectrochemical, and surface photovoltage measurements, the high photocatalytic activity can be attributed to the electrocatalytic properties of the Co₃O₄ cocatalyst and to the formation of a heterojunction at the BiVO₄–Co₃O₄ interface.