Cobalt oxide (Co₃O₄) nanoparticles were synthesized using the sol–gel method and calcined at various temperatures ranging from 300 °C to 600 °C to investigate the influence of thermal treatment on their structural, thermal and optical properties. X-ray diffraction (XRD) analysis confirmed the successful formation of a pure cubic spinel Co₃O₄ phase with nanocrystalline features, belonging to the Fd3m space group. As the calcination temperature increased, the samples exhibited enhanced crystallinity and sharper and more intense diffraction peaks, indicating grain growth and improved structural ordering. FTIR analysis confirmed the presence of functional groups and chemical bonding in Co₃O₄. Thermogravimetric analysis (TGA) indicated the elimination of surface adsorbed species and residual organics during the initial stages, succeeded by the stabilization of a cubic spinel Co₃O₄ phase, which exhibits remarkable thermal stability without any additional phase transitions. UV–Vis diffuse reflectance spectroscopy (DRS) analysis showed that the Co₃O₄ displayed significant absorption in the visible region, consistent with their intrinsic narrow bandgap characteristics. Unlike earlier sol–gel-synthesized Co₃O₄ nanoparticles, the present work highlights improved phase purity and long-term stability, making the material more stable for advanced applications. Optimized temperature Co₃O₄ nanostructures have great potential for next-generation energy storage devices, photocatalytic dye degradation, oxygen evolution reaction (OER) electrocatalysis, gas sensing, and biomedicine. This research not only provides valuable insights into the temperature-dependent development of Co₃O, but also sets a comparative standard for durable and scalable synthesis methods that align with modern trends in sustainable nanomaterials.