Copper-incorporated zinc oxide (ZnO:Cu) nanoparticles with different amounts of Cu incorporation (Cu:Zn ratio at% of 1.25, 2.5 and 5) were synthesized for the first time by a room temperature precipitation technique without any subsequent post thermal treatment. Pure ZnO nanoparticles were also synthesized for direct structural and property-related comparison purposes. ZnO:Cu nanoparticles were thoroughly characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, vibrating sample magnetometer, UV-visible and photoluminescence spectroscopy. Detailed crystallographic investigation was accomplished through Rietveld refinement. ZnO:Cu nanoparticles exhibited room temperature ferromagnetic (RTFM) property. The origin of RTFM in ZnO:Cu nanoparticles has been investigated and this property was attributed to the substitutional incorporation of Cu ions into the ZnO lattice. The evidence for substitutional incorporation has been demonstrated with the positional changes in XRD peaks of ZnO and accompanying precise lattice parameter analyses by Rietveld refinement. Chemical analysis by XPS revealed incorporation of copper as Cu2+ ions. The Rietveld-based percent occupancy values for Zn sites imply certain limited substitution of the Cu2+ ions into the ZnO lattice with the increasing amount of Cu. Meanwhile, the magnetization enhanced with increasing Cu amount, due to the narrowing band gap, suggesting that RTFM characteristics are an intrinsic property of ZnO:Cu nanoparticles. This work gives insight into the origin of RTFM in ZnO nanoparticles and can be used to enhance ferromagnetism in diluted magnetic semiconductors. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.