Artificial photosynthesis is a critical challenge in moving towards a sustainable energy future. Photocatalytic generation of hydrogen peroxide from water and dioxygen (H₂O + \(\frac{1}{2}\)O₂ → H₂O₂, ΔG° = 117 kJ mol^–1) by sunlight is a promising strategy for artificial photosynthesis because H₂O₂ is a storable and transportable fuel that can be used directly for electricity generation. All previously reported powder photocatalysts, however, have suffered from low efficiency in H₂O₂ generation. Here we report that resorcinol–formaldehyde resins, widely used inexpensive polymers, act as efficient semiconductor photocatalysts to provide a new basis for H₂O₂ generation. Simple high-temperature hydrothermal synthesis (~523 K) produces low-bandgap resorcinol–formaldehyde resins comprising π-conjugated and π-stacked benzenoid–quinoid donor–acceptor resorcinol couples. The resins absorb broad-wavelength light up to 700 nm and catalyse water oxidation and O₂ reduction by the photogenerated charges. Simulated sunlight irradiation of the resins stably generates H₂O₂ with more than 0.5% solar-to-chemical conversion efficiency. Therefore, this metal-free system shows significant potential as a new artificial photosynthesis system.