This work focuses on the structural, morphological, optical, photocatalytic, antibacterial properties of pure CeO₂ nanoparticles (NPs) and graphene oxide (GO) based CeO₂ nanocomposites (GO-1/CeO₂, GO-5/CeO₂, GO-10/CeO₂, GO-15/CeO₂), synthesized using the sol–gel auto-combustion and subsequent sonication method, respectively. The single-phase cubic structure of CeO₂ NPs was confirmed by Rietveld refined XRD, HRTEM, FTIR and Raman spectroscopy. The average crystallite size was calculated using Debye Scherrer formula and found to increase from 20 to 25 nm for CeO₂ to GO-15/CeO₂ samples, respectively. The related functional groups were observed from Fourier transform infrared (FTIR) spectroscopy, consistent with the outcomes of Raman spectroscopy. The optical band gap of each sample was calculated by using a Tauc plot, which was observed to decrease from 2.8 to 1.68 eV. The valence state of Ce (Ce³⁺ and Ce⁴⁺) was verified using X-ray photoelectron spectroscopy (XPS) for CeO₂ and GO-10/CeO₂. The poisonous methylene blue (MB) dye was used to evaluate the photocatalytic activity of each sample in direct sunlight. The GO-15/CeO₂ nanocomposite showed the highest photocatalytic activity with rate constant (0.01633 min^–1), and it degraded the MB dye molecules by 100% within 120 min. The high photocatalytic activity of this material for degrading MB dye establishes it as an outstanding candidate for wastewater treatment. Further, these nanocomposites also demonstrated excellent antimicrobial activity against Pseudomonas aeruginosa PAO1.