Ga-N co-doped ZnO thin films with reduced bandgaps were deposited on F-doped tin-oxide-coated glass by radio-frequency magnetron sputtering at different substrate temperatures in mixed N2 and O2 gas ambient. We found that Ga-N co-doped ZnO films exhibited enhanced crystallinity compared to undoped ZnO films grown under the same conditions. Furthermore, Ga-N co-doping ensured enhanced N-incorporation in ZnO thin films as the substrate temperature is increased. As a result, Ga-N co-doped ZnO thin films exhibited much improved photoelectrochemical (PEC) response, compared to ZnO thin films. Our results therefore suggest that the passive co-doping approach could be a means to improve PEC response for bandgap-reduced wide-bandgap oxides through impurity incorporation.