Photoelectrochemical　(PEC)　water　splitting　is　an　appealing　approach　by　which　to　convert　solar　energy　into　hydrogen　fuel.　Polymeric　semiconductors　have　recently　attracted　intense　interest　of　many　scientists　for　PEC　water　splitting.　The　crystallinity　of　polymer　films　is　regarded　as　the　main　factor　that　determines　the　conversion　efficiency.　Herein,　potassium　poly(heptazine)　imide　(K-PHI)　films　with　improved　crystallinity　were　in　situ　prepared　on　a　conductive　substrate　as　a　photoanode　for　solar-driven　water　splitting.　A　remarkable　photocurrent　density　of　ca.　0.80　mA　cm(-2)　was　achieved　under　air　mass　1.5　global　illumination　without　the　use　of　any　sacrificial　agent,　a　performance　that　is　ca.　20　times　higher　than　that　of　the　photoanode　in　an　amorphous　state,　and　higher　than　those　of　other　related　polymeric　photoanodes.　The　boosted　performance　can　be　attributed　to　improved　charge　transfer,　which　has　been　investigated　using　steady　state　and　operando　approaches.　This　work　elucidates　the　pivotal　importance　of　the　crystallinity　of　conjugated　polymer　semiconductors　for　PEC　water　splitting　and　other　advanced　photocatalytic　applications.