ZnS nanowires (NWs)–graphene (GR) nanocomposites were successfully synthesized via an easy electrostatic self-assembly method. The results showed that ZnS NWs were well dispersed on the surface of the graphene nanosheets, forming ZnS–GR nanocomposites. The photoluminescence intensity of the ZnS–GR nanocomposites exhibited a clear quenching in comparison with that of the ZnS NWs. The ZnS–GR nanocomposites were used as photocatalysts for the degradation of methylene blue (MB) under UV light irradiation. After irradiation for 1 h, the degradation efficiency was 21.69% for ZnS NWs, 31.52% for ZnS–0.5 wt% GR, 58.69% for ZnS–3 wt% GR, 88.33% for ZnS–5 wt% GR and 80.74% for ZnS–7 wt% GR, indicating that ZnS–5 wt% GR nanocomposites had the highest photocatalytic activity. The kinetic rate constant of the ZnS–5 wt% GR nanocomposites was about 16 times higher than that of the ZnS NWs. The mechanism of the highly enhanced photocatalytic properties was attributed to the efficient separation of the photogenerated electron–hole pairs and the high specific surface area of graphene.