The search for a semiconductor with an optimal band gap of about 2.0 eV as a photocatalyst for efficient water splitting using solar energy is one of the noble missions of material science. Here, we prove that single- and few-layer ZrS2 have an optimal band gap of about 2.0 eV and enough potential differences to drive the hydrogen and oxygen evolution reaction based on first-principles calculation with the Tran-Blaha modified Becke Johnson potential. Therefore, they can be used as efficient photocatalysts for solar water splitting. Moreover, we take single-layer ZrS2 as an example of tuning the band edge positions by strain in order to further improve the efficiency of solar energy conversion. The results show that a tensile uniaxial strain of 4% can minimize the difference between the conduction band potential difference and the valence band potential difference and, thus, maximize the photocatalytic efficiency of monolayer ZrS2. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.