An efficient visible-light photocatalyst was successfully synthesized by surface-hybridizing graphitic carbon nitride (g-C3N4) using a small amount of cadmium sulfide (CdS) nanoparticles. The CdS/g-C3N4 nanocomposites were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, electrochemical impedance spectroscopy, and photocurrent-time measurement. The photocatalytic activity of the CdS/g-C3N4 nanocomposites was investigated by evaluating the degradation of Rhodamine B (RhB) under visible-light irradiation. The effects of the CdS content on the nanocomposites, initial RhB concentration, pH value of the investigated system, and dosage of CdS/g-C3N4 nanocomposites on the visible-light photocatalytic activity were systematically investigated. The results revealed that the visible-light photocatalytic activity of g-C3N4-based photocatalysts was significantly improved by surface-hybridization of a small amount of CdS nanoparticles, increased as both CdS content on the nanocomposites and dosage of CdS/g-C3N4 nanocomposites increased, while increased at first and then decreased as both initial RhB concentration and pH value of the investigated system increased. The visible-light photocatalytic mechanism of the CdS/g-C3N4 nanocomposites was discussed.