<p>The persistent presence of organic pollutants such as dyes, antibiotics, and pesticides in water sources poses a serious threat to human and ecological health. Photocatalysis, particularly using semiconductor materials, offers a green, efficient approach for the mineralization of these pollutants under light irradiation. Among various photocatalysts, titanium dioxide (TiO₂) has been widely studied, yet its performance is limited by its wide bandgap and fast electron–hole recombination. Graphitic carbon nitride (g-C₃N₄), a polymeric, metal-free semiconductor, has emerged as a promising co-catalyst for forming heterojunctions with TiO₂. In this comprehensive review, we systematically summarize and critically analyze the synthesis, structural properties, photocatalytic mechanisms, and pollutant degradation performance of g-C₃N₄–TiO₂ heterostructures. Special emphasis is placed on the degradation of common classes of pollutants: synthetic dyes, pharmaceutical antibiotics, and agricultural pesticides. The influence of heterojunction type, band alignment, synthesis method, surface area, and charge carrier dynamics is critically discussed. This review also highlights current challenges and provides future perspectives for the rational design of g-C₃N₄–TiO₂ composites in environmental remediation.</p>