A fast and template-free method is proposed for the preparation of nanocrystalline ZnS in the presence of 1-ethyl-3-methylimidazolium ethylsulphate, [EMIM][EtSO4], a room temperature ionic liquid (RTIL), under microwave irradiation for 4 min. Powder X-ray diffraction (XRD) studies show that the products are excellently crystallized in a cubic structure. Energy dispersive X-ray (EDX) spectroscopy investigations reveal that the products are extremely pure. The results obtained from scanning electron microscopy (SEM) demonstrate that nanocrystalline ZnS prepared in the presence of the RTIL are smaller than that of the sample prepared in water and the size of the nanoclusters decreases with increasing RTIL content of the media. Diffuse reflectance spectra (DRS) of the as-prepared ZnS show a blue shift relative to the bulk ZnS, which can be attributed to quantum confinement effect of nanocrystalline ZnS. A possible formation mechanism of the nanocrystalline ZnS in aqueous solutions of the RTIL is presented. The photocatalytic activity of the nanocrystalline ZnS towards photodegradation of methylene blue (MB) was investigated. The results demonstrate that the photocatalytic properties of the nanocrystalline ZnS prepared in the presence of the RTIL are better than that of the sample prepared in water.