TiO2/g-C3N4/Ag nanocomposites were prepared and used as highly efficient photocatalysts. TiO2 nanoparticles were first prepared using a sol-gel process, and titania herostructures with varying amounts of graphitic carbon nitride (g-C3N4) were created using a hydrothermal technique. Following impregnation in a silver nitrate aqueous solution employing sodium citrate, the TiO2/g-C3N4 heterostructures were decorated with Ag nanoparticles. The prepared materials exhibit strong photocatalytic activity in both oxidation and reduction reactions. Under UV and artificial solar illumination, the degradation of four model contaminants, methylene blue (MB), rhodamine B (RhB), 4-nitrophenol (4-NP), and chromium(VI) (Cr+6), was examined. With degradation percentages of up to 95% for MB and 89% for RhB, an optimal condition was determined, resulting in the effective removal of dye contaminants under UV light. Fast degradation rates for these dyes were also observed under artificial solar light. In addition, the TiO2/g-C3N4/Ag nanocomposites were successful during photocatalytic reduction of 4-NP and Cr+6 contaminants in water, converting these hazardous compounds to a useful industrial precursor (aminophenol) and a less toxic substance (Cr+3), respectively.