As one of the most significant rare earth oxides, the redox ability of cerium oxide (CeO2) has become the primary factor that has attracted considerable interest over the past decades. In the present study, irregular pentagonal CeO2 (S-CeO2) and different amounts of (1, 4, 8, and 12% Co) cobalt-doped CeO2 nanoparticles (Co-CeO2 NPs) with particle sizes between 4 and 13 nm were synthesized via the microwave-assisted synthesis method. The structural, optical, and morphological studies of S-CeO2 and Co-CeO2 were carried out using various techniques. The shifts in the conduction band and valence band were found to cause the reduction of the band gap energies of S-CeO2 and Co-CeO2 NPs. Moreover, the quenching of photoluminescence intensity with more Co doping showed the enhanced separation of charge carriers. The photocatalytic activities of S-CeO2 and Co-CeO2 NPs for methylene blue dye degradation, 4-nitrophenol reduction, and their photoantibacterial properties under visible-light irradiation were investigated. Findings showed that, due to the lower band gap energy (2.28 eV), more than 40% of both photocatalytic activities were observed for 12% Co-CeO2 NPs. On the other hand, higher antibacterial impact in the presence of light shows that the Co doping has a considerable influence on the photoantibacterial response of Co-CeO2. Therefore, microwave-assisted synthesized CeO2 and Co-CeO2 NPs have shown potential in photocatalytic dye degradation, chemical reduction, and photoantibacterial activities.