Nickel oxide (NiO) nanoparticles were formed using the chemical precipitation method. The effect of the calcination process on the structural parameters, optical bandgap, and photocatalytic performance was investigated. The structural characteristics were carried out using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), and scanning electron microscope (SEM). The XRD analysis reveals that the formed NiO crystallized in an fcc crystal structure and the calcination process influences the crystallite size, microstrain, dislocation density, and average surface area. For example, the smallest and largest particle sizes (19.13 nm and 27.63 nm) were achieved for the samples prepared at 800 °C for 4 h and 900 °C for 2 h, respectively. Based on the diffuse reflectance spectroscopy analysis, the energy bandgap has the lowest values (3.33 eV) for the prepared NiO that calcinated at 800 °C for 2 h compared with other samples. The formation of a Ni–O stretching vibra tion mode is revealed by FTIR, and the broadness of the absorption band confirms that the NiO samples are nanocrystals. The morphology of the prepared NiO reveals the formation of spherical nanoparticles for NiO calcinated at 700 °C, while dodecahedron-like shapes were observed for NiO calcinated at 800 and 900 °C. The photocatalytic performance of NiO nanoparticles as catalysts for the degradation of indigo carmine dye was investigated under ultraviolet–visible irradiation up to 3 h. The best degradation efficiency was found to be 76% for NiO calcinated at 800 °C for 4 h, which belonged to the smallest crystallite size of 19.13 nm, and the highest surface area of 47.02 m2g−1. The superior and excellent performance of this sample compared to other samples was confirmed by achieving the highest reaction rate constant (4.51 × 10−3min−1). The proposed photodegradation mechanism shows the importance of increasing the time required for the recombination process between the positive holes and the excited electrons, which is the best possible when using the optimum photocatalyst sample that was prepared at 800 °C for 4 h.