Amphiphilic block copolymer poly(isoprene-b-acrylic acid) (PI-b-PAA) was used to stabilize exfoliated graphene in water, allowing the immobilization of semiconductor CdS nanoparticles forming CdS • PI-b-PAA/graphene. Characterization using high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy proved the success of the preparation method and revealed the presence of spherical CdS. Moreover, UV-Vis and photoluminescence assays suggested that electronic interactions within CdS • PI-b-PAA/graphene exist as evidenced by the significant quenching of the characteristic emission of CdS by exfoliated graphene. Photoillumination of CdS • PI-b-PAA/graphene, in the presence of ammonium formate as a quencher for the photogenerated holes, resulted in the generation of hydrogen by water splitting, monitored by the reduction of 4-nitroaniline to benzene-1,4-diamine (>80 ± 4% at 20 min; 100% at 24 min), much faster and more efficient compared to when reference CdS • PI-b-PAA was used as the photocatalyst (<30 ± 3% at 20 min; 100% at 240 min). Moreover, Rhodamine B was photocatalytically degraded by CdS • PI-b-PAA/graphene, with fast kinetics under visible light illumination in the presence of air. The enhancement of both photocatalytic processes by CdS • PI-b-PAA/graphene was rationalized in terms of effective separation of holes and electrons, contrary to reference CdS • PI-b-PAA, in which rapid recombination of the hole-electron pair is inevitable due to the absence of exfoliated graphene as a suitable electron acceptor.