Solar hydrogen production via natural lake water was investigated via monoclinic bismuth vanadate (BiVO4) photocatalyst in a tandem photoelectrochemical cell (PEC), which was made up via integrating a PEC system with a dye-sensitized solar cell (DSSCs) in a series. Herein, the visible-light-driven BiVO4 photocatalysts were synthesized via a simple hydrothermal treatment reaction. A mathematical representation for optimizing the synthesizing condition of BiVO4, namely pH and the hydrothermal temperature was successfully generated via response surface methodology (RSM). The BiVO4 sample prepared at pH = 1.96 and hydrothermal treatment at 120 °C (Run 4) exhibited remarkable photocatalytic hydrogen performance of 9.52 mmol/h within 7 h. In addition, the observed photocurrent density and photoconversion efficiency were 5.66 mA/cm2 at 0.7 V and 0.056, respectively. The profound performance observed was attributed to the low photocharge carrier resistance and a more negative flat band potential for PEC water splitting system. This work demonstrates the feasibility of producing hydrogen from abundant natural lake water via BiVO4 photocatalyst. © 2019