In the present investigation, a novel ternary nanocomposite, MgO/g-C₃N₄/Bi₂O₃, was successfully fabricated via a wet impregnation technique. The photocatalytic efficacy of this hybrid system was thoroughly assessed for the degradation of various organic pollutants, namely the antibiotic amoxicillin (AMX), the pesticide chlorpyrifos (CPF), and the dye methylene blue (MB), under visible light irradiation in an aqueous medium. Extensive physicochemical characterizations were performed employing XRD, SEM, TEM, EDS, XPS, UV-DRS, PL, BET, and EIS analyses. XRD patterns confirmed the coexistence of crystalline phases corresponding to hexagonal g-C₃N₄, monoclinic Bi₂O₃, and cubic MgO, with their respective characteristic planes distinctly discernible in the composite. Morphological investigations using SEM and TEM illustrated a hybrid structure composed of nanosheets interspersed with nanorods. Optical studies revealed enhanced visible light harvesting and a reduced bandgap energy of 2.3 eV, promoting efficient photoexcitation. Under optimized conditions—10 ppm pollutant concentration, pH 6, 100 mg catalyst dosage, and 90 minutes of visible light illumination—the nanocomposite achieved remarkable degradation efficiencies of 96.2% (MB), 74.2% (CPF), and 62.7% (AMX), substantially surpassing the performances of its individual components (g-C₃N₄, Bi₂O₃, and MgO). Radical quenching experiments identified photoinduced holes (h⁺) and superoxide radicals (O₂·⁻) as the primary reactive species orchestrating the degradation pathways. Importantly, the photocatalyst exhibited excellent structural robustness and maintained high photocatalytic activity over three consecutive cycles, demonstrating superior reusability and stability. These findings establish the MgO/g-C₃N₄/Bi₂O₃ nanocomposite as a highly efficient, visible-light-responsive photocatalyst for the sustainable remediation of organic contaminants in aquatic systems.