Wholly　distinct　from　conjugated　polymers　which　are　featured　by　generic　charge　transfer　capability　stemming　from　a　conjugated　molecular　structure,　solid　nonconjugated　polymers　mediated　charge　transport　has　long　been　deemed　as　theoretically　impossible　because　of　the　deficiency　of　π　electrons　along　the　molecular　skeleton,　thereby　retarding　their　widespread　applications　in　solar　energy　conversion.　Herein,　we　first　conceptually　unveil　that　intact　encapsulation　of　metal　oxides　(e.g.,　TiO2,　WO3,　Fe2O3,　and　ZnO)　with　an　ultrathin　nonconjugated　polyelectrolyte　of　branched　polyethylenimine　(BPEI)　can　unexpectedly　accelerate　the　unidirectional　charge　transfer　to　the　active　sites　and　foster　the　defect　generation,　which　contributes　to　the　boosted　charge　separation　and　prolonged　charge　lifetime,　ultimately　resulting　in　considerably　improved　photoelectrochemical　(PEC)　water　oxidation　activities.　The　interfacial　charge　transport　origins　endowed　by　BPEI　adornment　are　elucidated,　which　include　acting　as　a　hole-withdrawing　mediator,　promoting　vacancy　generation,　and　stimulating　the　directional　charge　flow　route.　We　additionally　ascertain　that　such　charge　transport　characteristics　of　BPEI　are　universal.　This　work　would　unlock　the　charge　transfer　capability　of　nonconjugated　polymers　for　solar　water　oxidation.　The　nonconjugated　insulating　polymer　was　utilized　as　a　charge　transport　mediator　for　boosting　charge　migration　and　separation　over　metal　oxides　toward　solar　water　oxidation.　©　2024　American　Chemical　Society.