We have fabricated an efficient visible-light-sensitive Cu²⁺-grafted Ce-doped ZnO photocatalyst (Cu²⁺–Ce_xZn_1−xO) by adopting a metal ion doping and co-catalyst modification. Impurity states were formed below the conduction band (CB) edge in Ce_xZn_1−xO, and these impurity states induce the visible-light absorption. Ce_xZn_1−xO without a Cu²⁺-co-catalyst showed negligible visible-light activity due to the low reduction power of electrons in impurity levels. Surprisingly, Cu²⁺-modification over Ce_xZn_1−xO drastically increased its visible-light activity. Excited electrons in impurity states can transfer to the Cu²⁺-ions on the surface and form Cu²⁺/Cu⁺ redox couples, which cause the efficient oxygen reduction through a multi-electron reduction process. One of the striking features of the present study is that the metal doped semiconductors which were inactive due to their impurity states become efficient visible-light photocatalysts upon co-catalyst modification. The successful strategy used here for designing a highly active visible-light photocatalyst would provide numerous opportunities to develop an efficient metal-ion based visible-light photocatalyst.