We report the efficient visible-light-responsive photocatalysis of polyoxometalates (POMs) by engineering the lowest unoccupied molecular orbitals (LUMOs). By the introduction of vanadium atoms into the γ-Keggin-type phosphotungstate, a new V3d/W5d mixed LUMO appeared to afford a visible-light-responsive catalyst (I), which showed high photocatalytic activity for aerobic oxygenation of sulfides to sulfoxides.