Photoelectrochemical water splitting from photocathodes based on wide-gap Cu(In,Ga)S2 (CIGS) thin films modified with a CdS layer and Pt nanoparticles was investigated. CIGS films with various amounts of Ga were fabricated using spray pyrolysis followed by sulfurization. As analyzed using 0.1 M Na2SO4 (pH 9) as an electrolyte under illumination of simulated sunlight (AM 1.5G), both photocurrent densities and onset potentials of the photocathodes were gradually improved by an increase in the amount of Ga in the CIGS films up to a Ga/(In + Ga) ratio of 0.25 (Pt-CdS/CIGS(25)). Further inclusion of Ga in the CIGS film was detrimental for both photocurrent density and onset potential. The maximum photocurrent density of 6.8 mA cm-2 (at 0 V vs RHE) and the highest photocurrent onset potential of 0.89 V vs RHE were obtained by using the Pt-CdS/CIGS(25) photocathode. Achievements of a relatively wide interface band gap of the CIGS/CdS heterointerface and formation of relatively large grains in the Pt-CdS/CIGS(25) sample were likely to be responsible for such superior water reduction properties.