A chemically stable 2D microporous COF (PMCR‐1) was synthesized via the multicomponent Povarov reaction. PMCR‐1 exhibits a remarkable and long‐term stable photocatalytic H₂O₂ production rate (60 h) from pure and sea water under visible light. The H₂O₂ production is markedly enhanced when benzyl alcohol (BA) is added as reductant, which is also due to a strong π–π interaction of BA with dangling phenyl moieties in the COF pores introduced by the multicomponent Povarov reaction. Motivated by the concomitant BA oxidation to benzaldehyde during H₂O₂ formation, the photocatalytic oxidation of various organic substrates such as benzyl amine and methyl sulfide derivatives was investigated. It is shown that the well‐defined micropores of PMCR‐1 enable size‐selective photocatalytic oxidation.