Novel organic–inorganic composites composed of two visible light responsive semiconductors of graphitic carbon nitride (C3N4) and CdS were successfully synthesized via an “in situ” precipitation–deposition method. The C3N4–CdS heterostructures were fabricated by depositing CdS nanoparticles onto the surface of C3N4. The morphology and optical property of compsoites can be tuned by adjusting the mass ratio of C3N4–CdS, which determines the enhanced level of photocatalytic activity. The optimum activity of 0.7C3N4–0.3CdS photocatalyst is almost 20.5 and 3.1 times higher than those of individual C3N4 and CdS for the degradation of methyl orange, and 41.6 and 2.7 fold higher for the degradation of 4-aminobenzoic acid, respectively. Moreover, its activity is also much higher than those of C3N4–TiO2 and CdS–TiO2 composites, as well as N-modified TiO2. Of special significance is that the present C3N4–CdS composites exhibit high stabilities under illumination, in contrast with CdS. The enhancement in both performance and stability should be assigned to the effective separation and transfer of photogenerated charges originating from the well-matched overlapping band-structures and closely contacted interfaces. Our work highlights that coupling semiconductors with well-matched band energies provides a flexible route to improve the activity and stability of photocatalysts, and gives ideas for the design and synthesis of other highly active and stable materials.