Hydrogen has received extensive attention due to its high energy density and non-polluting combustion products. Hydrogen production in a clean and efficient way has become the research focus. Photovoltaic-electrolysis, photocatalytic, and photoelectrocatalytic water splitting can utilize abundant solar energy and water to generate hydrogen, which are promising hydrogen production technologies. Electrochemical reaction is the key step in the water splitting process, which directly determines the energy conversion efficiency of the whole system. Most of the previous studies focused on the development of catalytic materials. The influence of the electrolyte characters on the electrolytic performance was always neglected. Therefore, the effects and significance of electrolytes on water splitting are discussed in this review. According to the basic principles of water splitting, the influence of the electrolyte pH value and ion composition on the surface catalytic reaction and mass transfer was discussed. The fine tuning of the electrolyte will affect the reactions at the solid-liquid interface, and potentially improve the energy conversion efficiency and stability in different water-splitting systems. It is expected that this work will help to build efficient water splitting systems and provides useful guidance for the large-scale applications.