Photoelectrochemical　water　oxidation　by　polymeric　carbon　nitride　(PCN)　has　recently　received　increasing　scientific　interest.　Herein,　phosphorus　(P)　is　doped　into　PCN　(p-PCN)　films　through　in　situ　polymerization.　p-PCN　sheets　are　formed　and　vertically　aligned　on　a　conductive　substrate,　which　is　used　as　a　photoanode.　The　optimal　photocurrent　density　of　the　photoanode　is　≈120　μA　cm−2,　which　is　more　than　two　times　that　of　the　undoped　one.　The　improved　performance　is　accounted　as　the　introduction　of　P　in　PCN.　On　the　one　hand,　P　doping　increases　the　valence　potential　of　PCN　and　improves　the　ability　of　water　oxidation.　On　the　other　hand,　charge　transfer　is　improved　by　extra　charges　from　P.　©　2020　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim