The effective S‐scheme homojunction relies on the precise regulation of band structure and construction of advantaged charge migration interfaces. Here, the electronic structural properties of g‐C3N4 were modulated through meticulous polymerization of self‐assembled supramolecular precursors. Experimental and DFT results indicate that both the intrinsic bandgap and surface electronic characteristics were adjusted, leading to the formation of an in‐situ reconstructed homojunction interface facilitated by intrinsic van der Waals forces. The homojunction catalyst, composed of g‐C3N4 nanodots and ultra‐thin g‐C3N4 nanoflakes, exhibited a significant S‐scheme carrier separation mechanism, which enhances the utilization of electrons and holes. Consequently, under AM 1.5 light irradiation (~100 mW/cm2), the g‐C3N4 homojunction photocatalyst achieved a remarkable hydrogen evolution rate of 580 μmol h‐1. Furthermore, a reversed CH4 selectivity in CO2 reduction was observed, yielding 80.30 μmol g−1 h−1 with a selectivity of 96.86%, in contrast to the performance of bulk g‐C3N4, which produced only 2.22 μmol g−1 h−1 with the 15.69% CH4 selectivity. These findings not only highlight the significant potential of the g‐C3N4 homojunction photocatalyst for hydrogen production and CO2 reduction but also propose a superior and effective strategy for optimizing the structural properties of g‐C3N4, which are crucial for the design of photocatalytic reactions.

g‐C3N4 S‐Scheme Homojunction through van der Waals Interface Regulation by Intrinsic Polymerization Tailoring for Enhanced Photocatalytic H2 Evolution and CO2 Reduction