A full‐spectrum (300–700 nm) responsive porphyrin supramolecular photocatalyst with a theoretical solar spectrum efficiency of 44.4% is successfully constructed. For the first time, hydrogen and oxygen evolution (40.8 and 36.1 µmol g−1 h−1) is demonstrated by a porphyrin photocatalyst without the addition of any cocatalysts. The strong oxidizing performance also presents an efficient photodegradation activity that is more than ten times higher than that of g‐C3N4 for the photodegradation of phenol. The high photocatalytic reduction and oxidation activity arises from a strong built‐in electric field due to molecular dipoles of electron‐trapping groups and the nanocrystalline structure of the supramolecular photocatalyst. The appropriate band structure of the supramolecular photocatalyst adjusted via the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the porphyrin gives rise to thermodynamic driving potential for H2 and O2 evolution under visible light irradiation. Controlling the energy band structure of photocatalysts via the ordered assembly of structure‐designed organic molecules could provide a novel approach for the design of organic photocatalysts in energy and environmental applications. A full‐spectrum dual‐function porphyrin supramolecular photocatalyst for hydrogen and oxygen evolution from water is achieved. The theoretical solar spectrum efficiency of self‐assembled tetra(4‐carboxylphenyl)porphyrin is 44.4%. The high activity of photocatalytic reduction and oxidation comes from the strong built‐in electric field due to molecular dipoles of electron‐trapping groups and the nanocrystal structure of the supramolecular photocatalyst.