The direct Z-scheme photocatalysts for overall water decomposition have aroused much concern on account of their strong redox ability and efficient separation of photogenerated electron-hole pairs. In the present work, we have constructed the two dimensional (2D) van der Waals (vdW) MoSe2/SnS2 heterojunction and investigated its electronical and optical properties by applying hybrid functional calculations. The calculated band structures have implied that the MoSe2/SnS2 heterostructure as a direct Z-scheme photocatalyst can make the best of visible light. The induced built-in electric field can effectively improve the separation efficiency of the photoinduced carriers. Moreover, compared with the MoSe2 and SnS2 monolayers, the absorption intensity of MoSe2/SnS2 heterojunction is reinforced in the visible light range. Therefore, MoSe2/SnS2 nanocomposite shows a bright application prospect as a direct Z-scheme visible-light-driven photocatalyst for overall water splitting. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.