Two-dimensional van der Waals (vdW) heterostructures were constructed using MoSSe and XN (X = Ga, Al) via density functional calculations to serve as water splitting photocatalysts. Both the MoSSe/GaN and MoSSe/AlN heterostructures are energetically, dynamically and thermally stable. Interestingly, they exhibited type-II band structures, indicating the ability to continuously separate photogenerated electrons and holes. They also have appropriate band edge positions for redox reaction potentials of water splitting at pH 0. Remarkably, the MoSSe/GaN vdW heterostructure possessed excellent carrier mobility for holes along both the transport directions. Besides, the charge transfer between the MoSSe and XN layers induced a strong built-in electric field, which further separated the photogenerated carriers. In addition, the MoSSe/GaN and MoSSe/AlN vdW heterostructures exhibit good optical absorption ability toward solar irradiation. All these excellent properties render the MoSSe/GaN and MoSSe/AlN heterostructures are high-efficiency photocatalysts for water splitting.