Graphitic carbon nitride (g-C₃N₄) photocatalysts were synthesized via thermal polymerization of urea in the presence of 1,3,5-trihydroxybenzene (THB) as a molecular modifier. The incorporation of 10 mg THB resulted in enhanced visible-light-driven photocatalytic performance for dye decolorization in aqueous solution. Under 450 nm irradiation, the modified g-C₃N₄ (T10) achieved over 90% degradation of Orange II within 60 min, with a pseudo-first-order rate constant approximately five times higher than that of pristine g-C₃N₄. Kinetic analysis based on the Langmuir–Hinshelwood model confirmed improved charge transfer efficiency and reduced electron–hole recombination. In contrast, Rhodamine B degradation exhibited substrate-dependent behavior, indicating that surface modification influences adsorption characteristics and interfacial reaction pathways. The enhanced performance toward Orange II is attributed to extended π-conjugation, improved visible-light absorption, and surface hydroxyl functionalities introduced by THB, which promote dye adsorption and reactive oxygen species formation. These findings demonstrate that molecular-level modification of g-C₃N₄ using aromatic hydroxyl compounds provides an effective strategy to tailor photocatalytic selectivity and improve visible-light-driven wastewater treatment performance.