Water purification by eco-friendly and cost-effective method is a challenge for scientists in the current era. The present study reports a facile and cost-effective green synthesis method of zinc oxide nanoparticles (ZnO-NPs) from leaves extract of Syzygium cumini plant. UV–visible spectroscopy verified the synthesis of nano-catalyst ZnO-NPs. X-ray diffraction technique and scanning electron microscopy explored the formation of pure crystalline hexagonal and spherical NPs. Fourier-transform infrared spectroscopy analysis ascertains the existence of flavonoids, phenolic acids, enzymes and steroids in leaves extract of S. cumini to reduce zinc slat into ions during synthesis and hence capping of the NPs. The ZnO-NPs were analysed for seed germination activity which showed enhancement in seed germination of Pennisetum glaucum seeds. The synthesised NPs were likewise utilised as nano-catalyst for degradation and removal of Rhodamine B (RhB) dye. Moreover, the effect of light irradiation time, temperature and pH was also investigated for optimisation of best conditions for degradation of dye. The green synthesised ZnO-NPs showed 98% degradation of RhB dye and purified the dye polluted water and, therefore, is an emerging catalyst for dye degradation.