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Photocatalytic conversion of biomass-derived compounds into valuable chemicals represents an important advancement for solar-energy conversion. However, conventional semiconductor photocatalysts, composed of multiple elements, exhibit nonuniform catalytic surfaces, complicating catalyst design and mechanistic understanding. Here, we present single-element semiconductors, red phosphorus (RP), to facilitate visible-light-driven Diels-Alder transformations of biomass-derived compounds into high-value products in moderate to good yields. The electron-rich phosphorus atoms effectively adsorb p-toluquinone and activate C-H bonds, leading to the formation of a cationic intermediate via photogenerated holes that promote Diels-Alder electronic matching with a diene into a cationic cyclized adduct. Accompanied by an O2-mediated process, wherein O2 accepts conduction-band electrons and transfers them to the cationic cyclized adduct, the D-A transformation is accomplished without requiring any sacrificial agents. This approach operates under milder and greener conditions, advancing the biomass conversion and highlighting RP's potential in organic synthesis. Additionally, the single-element photocatalyst allows for valuable insights into mechanistic studies.