Monodispersed and ultrastable colloidal ZnS nanospheres (NPs) composed of tiny nanoparticles were successfully synthesized by using a limited ligand-induced in situ aggregation strategy. With such a strategy, the whole size as well as the particle size of those ZnS NPs could be tuned simultaneously by appropriately varying the reaction conditions. Three representative ZnS NP samples with different sphere sizes and particle sizes were thus obtained, which were all proven to possess rather large surface areas, robust structures and excellent colloidal stability. Furthermore, the photocatalytic activities of the as-prepared ZnS NPs toward the photodegradation of eosin B, methylene blue and their binary mixture were explored respectively. An interesting size-dependent degradation performance associated with the ZnS NPs was observed in all the photodegradation cases. Finally, their degradation mechanism was fully elucidated according to the control experiments under different atmospheres in combination with the related energy level information. We believe that the control strategy for tuning the fine and whole structures of spherical nanostructures in a synergistic manner together with the structure-dependent photodegradation performance revealed herein will definitely benefit the fabrication of highly efficient photocatalysts as well as the nanocomplexes with hierarchical architectures.