Abstract In this study, we used an environmentally friendly green method to synthesize NiO nanoparticles (NiO NPs) with an extract from Persea americana seeds for use as photocatalysts in the photodegradation of free cyanide (FCN). The characteristics of the NiO NPs were determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), ultraviolet–visible (UV–Vis) analysis, and Fourier transform infrared (FTIR) spectroscopy. The XRD results indicated a cubic-type structure with space group F m 3 m , No. (225). Morphological analysis conducted using TEM showed that the NiO NPs had rhombohedral and spherical shapes with slight agglomeration. The particle size calculated based on the TEM results was 11 nm. EDS analysis confirmed the elemental constituents of the samples with Ni and O atoms. The phase purity and chemical bonds in NiO were confirmed using FTIR spectroscopy analysis. UV–Vis analysis indicated that the NiO NPs samples exhibited good optical properties with a well-defined peak at 338.9 nm. The energy band gap E g obtained for NiO NPs based on the optical analyses was 3.75 eV. The NiO NPs performed well as a photocatalyst under UV light during the photodegradation of FCN, where the maximum photocatalytic oxidation efficiency was 84% after irradiation for 30 min. The FCN photodegradation mechanism was elucidated based on the photocatalytic oxidation of compounds by oxygen with semiconductors.