Graphitic carbon nitride (g-C₃N₄) has emerged as a promising metal-free polymeric semiconductor with immense transformative potential in solar-driven energy conversion and environmental remediation, underpinned by its thermodynamically favorable band structure for key redox reactions, exceptional chemical and thermal stability, low-cost scalable synthesis, and adjustable visible-light responsiveness. However, its practical industrial exploitation remains severely hampered by three intrinsic bottlenecks: ultrafast recombination of photogenerated electron-hole pairs that drastically reduces quantum efficiency, inadequate visible-light absorption region that limits full-spectrum solar utilization, and a scarcity of active surface sites that restricts overall reaction performance. To overcome these long-standing intrinsic bottlenecks, biomass-based functionalization has emerged as a rapidly growing research frontier, offering a sustainable, low-cost alternative to conventional high-input chemical modification strategies. As the most abundant renewable carbon feedstock on Earth, biomass providing a uniquely versatile and green platform for the rational engineering of g-C₃N₄. In this review, we systematically summarize state-of-the-art recent advances in biomass-engineered g-C₃N₄ photocatalysts, elaborate their core regulatory functions and underlying enhancement mechanisms, and provide critical theoretical references for future research in this rapidly evolving field.