Herein, we report the synthesis of Zinc sulfide quantum dots (ZnS QDs) by chemical precipitation method using air‐stable complex of hydrazine hydrate‐sulfur is reported. The hydrazine hydrate‐sulfur complex plays a vital role in the growth of ZnS QDs. The structural studies confirm that ZnS QDs are highly stable, spherical in shape, monodisperse, and exhibit wurtzite structure. The estimated particle size is in the range of 3–6 nm. Furthermore, the photocatalytic activity of the ZnS QDs for the photodegradation of methyl orange is explored. The ZnS QDs exhibit interesting size dependent degradation performance. The photodegradation efficiency increases with increase in the size of ZnS QDs and the highest observed photodegradation efficiency is 66.31%.