Covalent　organic　frameworks　with　unique　π　architectures　and　pores　could　be　developed　as　photocatalysts　for　transformations.　However,　they　usually　form　π-stacking　layers,　so　that　only　surface　layers　function　in　photocatalysis.　Here　we　report　a　strategy　for　developing　vertically　expanded　frameworks　to　expose　originally　inaccessible　active　sites　hidden　in　layers　to　catalysis.　We　designed　covalently　linked　two-dimensional　cobalt(II)　porphyrin　layers　and　explored　coordination　bonds　to　connect　the　cobalt(II)　porphyrin　layers　with　bidentate　ligands　via　a　three-component　one-pot　polymerization.　The　resultant　frameworks　expand　the　interlayer　space　greatly,　where　both　the　up　and　down　faces　of　each　cobalt(II)　porphyrin　layer　are　exposed　to　reactants.　Unexpectedly,　the　vertically　expanded　frameworks　increase　skeleton　oxidation　potentials,　decrease　exciton　dissociation　energy,　improve　pore　hydrophilicity　and　affinity　to　water,　and　facilitate　water　delivery.　Remarkably,　these　positive　effects　work　collectively　in　the　photocatalysis　of　water　oxidation　into　oxygen,　with　an　oxygen　production　rate　of　1155　μmol　g−1　h−1,　a　quantum　efficiency　of　1.24　%　at　450　nm,　and　a　turnover　frequency　of　1.39　h−1,　which　is　even　5.1-fold　as　high　as　that　of　the　π-stacked　frameworks　and　ranks　them　the　most　effective　photocatalysts.　This　strategy　offers　a　new　platform　for　designing　layer　frameworks　to　build　various　catalytic　systems　for　chemical　transformations.　©　2024　Wiley-VCH　GmbH.