Photocatalytic　overall　water　splitting　is　an　ideal　pathway　to　store　solar　energy,　and　produce　clean　and　sustainable　hydrogen　fuel.　However,　exploring　photocatalytic　materials　with　high　efficiency　for　overall　water　splitting　remains　a　challenge.　Herein,　crystalline　carbon　nitride　(CCN)　was　in　situ　grown　on　LaOCl　to　improve　the　performance　for　overall　water　splitting.　Owing　to　the　enhanced　photogenerated　carrier　transfer　by　crystalline　carbon　nitride,　and　the　promoted　spatial　separation　of　carriers　via　the　built-in　electric　field　between　CCN　and　LaOCl,　the　optimized　CCN/LaOCl　composite　catalyzes　the　decomposition　of　H2O　into　H2　and　O2,　with　evolution　rates　of　60.6　and　28.1　μmol　h−1,　respectively,　prominently　surpassing　those　of　most　of　the　reported　carbon　nitride-based　photocatalysts.　This　work　is　anticipated　to　provide　new　perspectives　for　the　rational　design　and　preparation　of　CCN-based　hybrid　photocatalysts　for　solar　water　splitting.　©　2022　The　Royal　Society　of　Chemistry.