Fe-substituted NiO nanoparticles (Ni1−xFexO2) (x= 0.00, 0.03, 0.05, 0.07, and 0.09) were synthesised using ball-milling method followed by vacuum sintering and studied for their microstructural and magnetic properties. All the samples show a cubic structure with a crystallite size of 17–26 nm. Secondary phases related to Fe2O3 were identified from XRD, indicating that the observed ferromagnetism may be due to the dopants. The doping of Fe ions does not cause any significant variation in the crystal structure. The pure NiO nanoparticles were weak ferromagnetic in nature, and Fe-doped NiO nanoparticles exhibit room temperature ferromagnetism. The strength of magnetization increased from 0.57 to 0.94 emu/g with an increase of Fe concentration from 3 to 7 at.%, respectively. A high coercive field of 173 Oe was observed for the Ni1−xFexO2 nanoparticles at x= 0.09. The results and the process of development of ferromagnetism at room temperature in the Ni1−xFexO2 nanoparticles were presented and discussed in detail. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.