Single　two-dimensional　(2D)　GaS　and　GaSe　were　studied　as　photocatalysts,　yet　the　overall　performance　is　limited　by　the　low　optical　absorption　and　inefficient　separation　of　photogenerated　electron-hole　pairs.　Constructing　van　der　Waals　(vdW)　heterostructures　is　an　ideal　way　to　overcome　the　deficiency　of　single　2D　gallium　chalcogenides.　This　work　unravels　that　gallium　chalcogenides/arsenene　(GaX/As,　X　=　S,　Se)　are　the　promising　vdW　heterostructures　that　show　significantly　improved　photocatalytic　performance　by　means　of　first-principles　calculations.　The　GaX/As　heterostructures　possess　suitable　band　alignment　and　bandgap　satisfying　the　requirements　for　photocatalysts.　Contrary　to　the　pristine　monolayers,　the　Se0.5GaS0.5/As　and　S0.5GaSe0.5/As　heterostructures　undergo　indirect-direct　bandgap　transition　by　varying　the　interlayer　distances;　moreover,　they　exhibit　high　carrier　mobility　(∼2000　cm2　V−1　s−1　for　electrons)　and　transport　anisotropy,　efficiently　facilitating　the　migration　and　separation　of　photogenerated　electron-hole　pairs.　Finally,　all　GaX/As　heterostructures　show　significantly　enhanced　optical　absorption　beyond　the　isolated　GaX　monolayers　under　visible-light　irradiation.　These　extraordinary　properties　render　GaX/As　heterostructures　as　competitive　photocatalysts　for　water　splitting　to　produce　hydrogen.　©　2018　Hydrogen　Energy　Publications　LLC