Green synthesis is a prominent synthesis route to prevent detrimental effects exhibited by the traditional methods for nanoparticle preparation on a small or large scale. Moreover, the green nanomaterials possess controllable properties, such as shape and size ascribed to the electrical, optical, magnetic, and catalytic properties of the metal nanoparticles. These characteristics make nanomaterials (NMs) unique in addressing environmental issues. This review chapter discusses the preparation of different nanoparticles, such as metal or metal oxide-based nanoparticles using the green pathway and their photocatalytic applications in wastewater treatment.