Graphitic carbon nitride (g-C3N4) is a promising, earth-abundant visible light photocatalyst characterized by its unique two-dimensional structure, excellent chemical stability, and tunable electronic properties. Despite its potential, pristine g-C3N4 faces challenges, such as the rapid recombination of photogenerated electron-hole pairs, which reduces photocatalytic efficiency. To overcome this limitation, g-C3N4 has been extensively coupled with various functional materials to enhance its performance. This chapter explores six primary categories of g-C3N4-based nanocomposites, classified according to their photocatalytic mechanisms: metal-free heterojunctions, single metal oxide or sulfide heterojunctions, composite oxides, halide heterojunctions, noble metal heterostructures, and complex hybrid systems. The synthesis methods of these nanocomposites, structural designs, and diverse applications are reviewed, with an emphasis on mechanisms behind photocatalytic activity augmentation. The primary modes of action underpinning enhanced performance include p-n junctions, Schottky junctions, surface plasmon resonance (SPR), photosensitization, and further interfacial phenomena. Furthermore, the discussion brings to bear the applications of g-C3N4-based nanocomposites in environmental and energy-related fields: pollutant degradation, hydrogen production, carbon dioxide reduction, water disinfection, and energy storage in supercapacitors. The discussion ends by an elaboration of the current challenges and future aspects in the field of g-g-C3N4-based nanomaterials, guiding them into a broader sphere of sustainable technologies. Through this chapter, a holistic overview of the g-C3N4-based nanocomposites will be framed and their pertinence in relation to the looming challenges in environment and energy will be elucidated.