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The opto-photocatalytic applications of Mn-doped NiO nanoparticles (NPs) are discussed. A facile synthesis of NiO doped with different Mn contents is performed and studied using experimental techniques. The structural properties and microstructural parameters of Mn:NiO NPs were examined by X-ray diffraction patterns. The average crystallite sizes of the pure NiO and 1.0-, 2.5-, 5.0-, 10-wt.% Mn-doped NiO NPs were found to be 25–19 nm by Williamson–Hall plots and 23–18 nm by the Debye–Scherer method. The crystallite size decreases with increasing doping concentration in the NiO host lattice. The optical bandgaps are found to be 3.53, 3.46, 3.39, 3.24, and 3.12 eV for pure NiO and 1.0-, 2.5-, 5.0-, and 10-wt.% Mn:NiO NPs, respectively, using the Kubelka–Munk function. X-ray photoelectron spectroscopy spectra of 2.5-wt.% Mn:NiO NPs were recorded for Mn, Ni, and O to determine the oxidation states. The photocatalytic performances of pure NiO and Mn:NiO NPs are examined. The 2.5-wt.% Mn:NPs exhibit the best photocatalytic performance owing to the greatest activation sites. The mechanism of the photocatalytic decolorization reaction is also investigated. The results demonstrate the high ability of Mn:NiO NPs as photocatalysts for methylene blue dye degradation, with 92% removal.