Anise and ginger extracts were used as green reducing agents to synthesize high-crystallinity nickel oxide nanoparticles (NiO NPs). The crystalline nature of the synthesized NiO NPs was confirmed by X-ray diffraction (XRD) analysis. Furthermore, the morphology of the NiO NPs was examined using a scanning electron microscope. The particle sizes of the NiO NPs, determined by field emission scanning electron microscopy and XRD, were in the ranges of 15.15–17.51 nm and 35.433–35.924 nm, respectively. Energy-dispersive X-ray spectroscopy, together with the XRD patterns, verified the high purity of the synthesized NiO NPs. The as-prepared NiO NPs exhibited excellent photocatalytic activity toward the photodegradation of methylene blue dye under direct sunlight irradiation. This work represents a positive step toward the use of green photocatalysts to reduce harmful dyes in water. Furthermore, the half-maximal inhibitory concentration (IC₅₀) values of NiO NPs synthesized using anise and ginger extracts were 68.82 and 61.5 μg/mL, respectively, as determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide assay on SW480 cells.