This study presents the fabrication of a photovoltaic-photoelectrochemical-integrated Cu(In, Ga)Se2 (CIGS)-related co-planar device that resulted in an onset potential (V
onset) of over 1.23 V to achieve photoelectrochemical water splitting without external bias. Therefore, the utility of this device was indicated for unassisted water splitting reaction. Moreover, the effects of the open-circuit voltage of the CIGS solar cell part on the photocurrent density and V
onset of the fabricated water splitting device were investigated. These results suggest that the applying the reverse bias owing to V
oc of the CIGS solar cell part influences the space charge layer at the surface of the CIGS photoelectrode. This effect led to the formation of an inversion layer, suppressing surface recombination on the CIGS photoelectrode and contributing to an increase in the photocurrent density. The results represent a preliminary step toward realizing potential applications of the CIGS photovoltaic-photoelectrochemical device for the unassisted water splitting reaction.