Abstract: Heptazine-based carbon nitride is a polymeric carbon nitride consisting primarily of heptazine units linked by bridging nitrogen atoms. As a metal-free, visible-light-responsive photocatalyst, it has garnered extensive attention in recent years due to its low cost, facile synthesis, excellent chemical stability, unique electronic band structure, and environmental friendliness. Nevertheless, its practical photocatalytic performance remains severely limited by intrinsic drawbacks, including rapid charge-carrier recombination, low specific surface area, and insufficient visible-light utilization. Morphology regulation has therefore emerged as an effective strategy to address these challenges. This review systematically summarizes recent progress in the dimensional engineering of g−C₃N₄, encompassing zero-dimensional (0D) quantum dots, one-dimensional (1D) nanorods and nanotubes, two-dimensional (2D) nanosheets, and three-dimensional (3D) porous and hierarchical architectures. It critically analyzes the influence of various synthesis strategies—including top-down and bottom-up approaches, template methods, supramolecular self-assembly, and hydrothermal reassembly—on the material's microstructure, electronic band structure, and surface-active sites. The results indicate that dimensional reduction to low-dimensional structures significantly enhances light absorption and charge carrier separation efficiency via quantum confinement effects, shortened charge transport channels, and ultrathin architectures. Conversely, three-dimensional structures integrate multi-dimensional advantages, improving structural stability and mass transfer capabilities while maintaining high activity. To address current challenges such as poor synthetic reproducibility, unclear structural evolution mechanisms, and ambiguous structure–activity relationships, this review proposes the development of cross-scale synergistic structures, the application of in situ/operando characterization techniques, and the integration of theoretical calculations with experimental approaches. These strategies aim to guide the rational design and practical engineering application of high-performance g−C₃N₄ photocatalysts.