The application of solar-driven photocatalytic processes shows considerable potential for renewable energy production and environmental remediation. Graphitic carbon nitride (g-C₃N₄) has emerged as a highly promising metal-free photocatalyst due to its outstanding electronic structure and physicochemical properties. However, the intrinsic constraints of pristine g-C₃N₄, such as limited visible light absorption range, high recombination rates of photogenerated charge carriers, and a scarcity of active sites, have significantly hindered its photocatalytic performance and practical implementations. Recent studies have demonstrated that defect engineering can substantially mitigate these issues by enhancing both light absorption and charge separation efficiency, thereby improving photocatalytic performance. This review provides a comprehensive overview of intrinsically defective g-C₃N₄-based materials, focusing on the types of intrinsic defects, their modification strategies, and the recent advancements in the field. It also highlights the diverse applications of defect-modified g-C₃N₄, including wastewater remediation, hydrogen evolution, CO₂ conversion, NO removal, nitrogen fixation, photocatalytic disinfection, and H₂O₂ production. Finally, the current challenges and future perspectives are discussed of g-C₃N₄-based photocatalytic materials, offering insights and practical guidance for the development of advanced g-C₃N₄-based photocatalysts.