Graphitic carbon nitride (g-C3N4), a conjugated polymer semiconductor, has drawn increasing attention in environmental photocatalysis over the past decade, owing to its highly chemical stability, low cost and suitable electronic structure with a mild energy gap (similar to 2.7 eV). Whereas, its practical applications in wastewater purification are still faced with huge challenges, such as insufficient visible light absorption, low electronic conductivity, poor surface area and fast recombination of photoinduced charge carriers. The modification of g-C3N4 by coupling with other nanomaterials or forming unique nanostructures have been confirmed as valuable strategies. In this review, we give a comprehensive introduction about the recent developments in engineering g-C3N4 by constructing heterostructures or diverse morphologies to improve its photodegradation performance for the photocatalytic degradation of persistent organic pollutants. Such heterojunctions contain metal/g-C3N4, g-C3N4/C-based material and many other binary or ternary composites, and these special morphologies normally include nanorods, nanowires, nanotubes, nanosheets, porous architectures and other different tunable nanostructures. Several enhanced photocatalytic mechanisms for these nanohybrids under visible light irradiation have been explicated in detail, and future perspectives are also concluded in this review.