CdS–graphene (GR) and CdS–carbon nanotube (CNT) nanocomposites prepared by a hydrothermal method were studied as photocatalysts for the evolution of hydrogen and the degradation of methyl orange (MO) under visible-light irradiation. The incorporation of GR or CNT into CdS significantly enhanced the photocatalytic activities for both reactions. The photocatalytic activities depended on the mass ratio of CdS : GR or CdS : CNT in the nanocomposites. Under optimized mass ratios, the CdS–GR was more efficient than the CdS–CNT. Our characterizations suggested that CdS nanoparticles of ∼35 nm in size were dispersed on the graphene sheets or CNT surfaces in the nanocomposites. Significant band-gap narrowing was observed due to the incorporation of GR or CNT into CdS, indicating the strong interactions between CdS and GR or CNT. The transient photocurrent response studies suggested a more efficient separation of photogenerated electrons and holes. The stronger interaction or larger contact interface between CdS and GR was proposed to accelerate the transfer of photogenerated electrons from CdS to GR more efficiently, resulting in higher photocatalytic activities of the CdS–GR composite.