NiO/AC and Co-doped NiO/AC photocatalysts with different Co concentrations (3, 5, 7.5, and 10%) were synthesized via the hydrothermal technique. The catalysts are labeled as NiO/AC, Co(3%):NiO/AC, Co(5%):NiO/AC, Co(7.5%):NiO/AC, and Co(10%):NiO/AC. In the presence of these catalysts, photocatalytic degradation measurements of methylene blue (MB) were performed and it was determined that the catalyst with the highest degradation efficiency was Co(3%):NiO/AC. In the presence of Co(3%):NiO/AC photocatalyst, the effect of different parameters such as dye concentration, photocatalyst amount, and system pH on the photocatalytic degradation reaction was investigated. Moreover, the effect of Co-doped metal on the crystalline size and bandgap of the hexagonal NiO/AC was studied. It was observed that the crystalline size of Co(3%):NiO/AC photocatalyst is greater than that of NiO/AC, while the energy bandgap of Co(3%):NiO/AC photocatalyst is narrower than that of the NiO/AC.