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Nanomaterials with collective optical and magnetic properties are called smart or functional materials and have promising applications in many fields of science and technology. Undoped and Co-doped ZnAl₂O₄ were prepared using a co-precipitation-assisted hydrothermal method. A systematic investigation was carried out to understand the effects of the Co concentration on the crystalline phase, morphology, and optical and magnetic properties of Co-doped ZnAl₂O₄. X-ray diffraction confirmed the cubic spinel structure with the Fd3m space group, and there was no impurity phase. X-ray photoelectron spectroscopy of Co-doped ZnAl₂O₄ confirmed the existence of Zn, Al and O, and the Co in the optimized sample of ZAO-Co-3 confirmed the oxidation state of cobalt as Co²⁺. Transmission electron microscopy of pure and Co-doped ZnAl₂O₄ revealed micro-hexagons and nanosheets, respectively. The optical absorption results showed that the bandgap of ZnAl₂O₄ decreased with increasing Co concentration. The hysteresis loop of Co-doped ZnAl₂O₄ revealed clear ferromagnetic behavior at room temperature. The as-prepared materials are suitable for energy storage applications, such as in supercapacitors and fuel cells. This work aims to focus on the effect of cobalt ions in different concentrations on structural, optical and magnetic properties.