Divalent-metal-ion (Zn2+ or Mg2+)-doped GaN with d(10) electronic configuration was found to make a high and stable photocatalyst for overall water splitting when combined with RuO2 as a promoter, whereas neither undoped nor tetravalent-metal-ion (Si4+ or Ge4+)-doped GaN showed photocatalytic activity upon loading of RUO2. The present work clearly demonstrated the marked effects of dopants on photocatalytic performance of the metal nitride GaN for water splitting.